On the ground – When you push block something on the ground and let go of block you will eventually be 100% free meaning you won’t have any guard stiffness after a while. You can use that to escape out of almost any trap on the ground since jumping is very fast (2-3 frames to be off the ground). However, there are moves that can stop your jump, for instance Strider’s trap can stop your jump because the orbs prevent you from being off the ground for too long so you’ll get hit before you fully escape high into the air. However if you continue to block instead of trying to do something you’ll just keep blocking and won’t be guard broken if you were being hit by a Proton Cannon or something. When you get out of block stun you can do anything, not just super jump, a funny thing to try is using instant supers and DHC’s to punish something while it’s still going.

In Normal-Jump mode – There are various times you can be in normal jump mode, not just after normal jumping up. For instance if you tech-hit or if you unfly and you don’t have unfly, if you’re Dhalsim and you perform a drill while in SJ mode, and many other things! Anyway, when you’re in normal jump mode you can be easily guard broken and sometimes push blocking helps. Even if push blocking is your first action when you jump into the air you’ll still be vulnerable when you finish your push block animation (well, its just one frame held for a second or two). So it’s easy to say that push blocking can be very dangerous unless you’re in a certain situation where the person can’t hit you or you can call an assist to protect you while you fall.

In Super-Jump mode – Usually push blocking in during your super jump just means that you will be able to throw the guy if he’s rushing you with attacks. However that’s not exactly true, you can be guard broken in super jump mode after a while and if you’re being bombarded with continuous attacks. For instance if you’re playing Magneto and he air dashes towards you and does his magic series and you push block it – he can go into a tempest for a little then into hail storm and you’ll be guard broken!

Summary

To perform an Advancing Guard, also known as Push-Block, just perform a Block or an Air Block and, while still in Block Stun, press any two of the main Attack Buttons.

Leveling Up Your Defense

While Blocking is the most fundamental building block of defense (no pun intended), the most important aspect of defense is the Advancing Guard. Characters in Marvel Vs. Capcom 3 have so many ways to overwhelm their opponent with offense and the best way to counter that is by using Advancing Guard.

What Advancing Guard does is, when activated while blocking, it will push your opponent a very large distant away from you no matter what move it is that you blocked. So even if Felicia performs a single Crouching Light Attack and you block that or Ryu throws a Hadoken at you and you block that or Sentinel performs a Jumping Hard Attack from the air and you block that, hitting two of the three main Attack Buttons will cause a "barrier" to appear around your character as they lean forward, and the opponent gets pushed away.

The benefits of this are obvious: it gets your opponent off of you and keeps them away. So when a character like X-23 is constantly pressuring you, you can keep them off using Advancing Guard. Advancing Guard must be learned. It is sometimes the only way to swing momentum. Without it, you'll likely be overwhelmed and get K.O.'ed almost instantly by rushdown characters.

Advancing Guard Limitations

However, Advancing Guard has some limitations. The first limitation is that it has no effect on Assist Characters. So if your opponent calls in Chun Li with her Gamma Assist, the Hyakuretsu Kyaku, and you start blocking one hit of the move and you perform an Advancing Guard, you cannot push Chun Li away. Also, your Point Character will not be pushed away either.

The reason your Point Character will not get pushed away is because, technically, characters who land the attack are the only ones who can actually be pushed away. Since Assist Characters are immune to being pushed away, neither character gets pushed away. However, there is another way to take advantage of this phenomenon. If you perform an attack that remains on the screen even after you leave, the new Point Character will not be affected by the Advancing Guard! For example, using Trish, perform a Round Harvest Hyper Combo. Once the blade starts hitting the enemy, normally if the opponent performs an Advancing Guard, Trish will get pushed away.

However, if you DHC to swap to another Point Character or even just perform a Cross-Over Attack to swap to a new Point Character, the blade from the Round Harvest will remain attacking the opponent even after Trish leaves. If the opponent performs an Advancing Guard while blocking the Round Harvest, your new Point Character will actually not be affected! Because the Round Harvest was performed by Trish, Trish is the only character who can technically be affected by the Advancing Guard! Since she's now an Assist Character and a new Point Character is in place, the Advancing Guard will have no affect on the new Point Character! So you can continue advancing on the opponent without fear of ever getting pushed back.

The other main limitation is that, once you perform an Advancing Guard, you are stuck in "Advancing Guard Delay." Advancing Guard lasts a very specific amount of time and, until that time passes, you cannot perform any other actions outside of blocking again. The Advancing Guard Delay is not very long, however, but it does lock you into place for a second.

So let's say Felicia comes up to you and performs a Crouching Light Attack canceled into a Medium Attack version of the Cat Spike. That sequence is not a true combo nor Block String, so if you blocked the Crouching Light Attack, you can recover before the Cat Spike and hit her with a quick move.

However, if you block the Crouching Light Attack from Felicia and perform an Advancing Guard, you will go into Advancing Guard Delay. So even though those two moves in that attack sequence used by Felicia is not a true Block String, thanks to Advancing Guard Delay you will be forced to also block the Cat Spike from Felicia. So by performing an Advancing Guard, you've essentially turned that sequence into a Block String. If you hadn't performed the Advancing Guard, you'd have been able to do something in between.

Strategy Corner
Eventually, you will run into some players who become adept at pushing you away via Advancing Guard every single time you touch them, which makes it very hard to stay in their face for a rushdown. However, by understanding the mechanics of the game, you can actually learn to try and bait Advancing Guards and punish accordingly.

For example, if you jump in on your opponent and attack and the perform an Advancing Guard on your Jump Attack after blocking, you can take advantage of this. High blocking a Jump Attack and performing an Advancing Guard has a command overlap with Back Dashing: holding Back and pressing two of the three Attack Buttons. So you can learn to actually jump at someone and do nothing and land with a quick attack instead. What happens is that the opponent will instinctively try and perform an Advancing Guard on your Jump Attack. But since you didn't do one, they will accidentally execute their Back Dash instead! So landing with an attack will allow you to hit them out of their Back Dash and start up a combo.

If you run up to the opponent while they are crouching, however, trying an Advancing Guard will not cause them to Back Dash. It will just cause them to perform a Crouching Attack depending on which two buttons they used to try their Push Block. However, if you run up to them and delay your attack for a brief second, you may be able to bait your opponent into trying an Advancing Guard and then can hit their move with a faster move before it comes out. Since Advancing Guard requires at least two buttons and, when two buttons are pressed at the same time, the stronger of the two buttons comes out, the fastest move the opponent can get as a result of a whiffed Advancing Guard is a Medium Attack. Delaying your attack and then pressing a Light Attack can cause them to whiff their Advancing Guard and then hitting their Medium Attacks or Hard Attacks before their move starts hitting.

Also, if you know your opponent is going to perform an Advancing Guard on you, one of the best ways to get in is to perform a move that will close the gap even after they perform the Advancing Guard. For example, with She-Hulk, if you attack the opponent with a Crouching Light Attack and you expect them to push you away with an Advancing Guard, cancel the move into your Hard Attack version of the Heavy Strike. That moves causes her to run forward and try to throw the opponent. So even though the opponent has pushed you back, the move itself will cancel out the backwards momentum of the Advancing Guard and still catch the opponent right when they recover from the Advancing Guard Delay.

You can even use moves that hit like Thor's Hard Attack Mighty Smash. Let's say you the enemy in the corner and you anticipate them Push Blocking your Crouching Light Attack. Cancel the Crouching Light Attack into a Mighty Smash and the backwards momentum from the Advancing Guard will be counteracted by the forward momentum of the Hard Attack Mighty Smash, which will make it so that the Mighty Smash lands right on the opponent. And since the Mighty Smash cannot be punished on Block, you stay in their face despite the Advancing Guard. This technique takes anticipation, but can be quite effective if you have moves that close the distance and are safe on block.

Air Advancing Guard

Also worth noting is that, when you perform an Advancing Guard during an Air Block, you "lock" yourself in the air for the entirety of the Advancing Guard Delay. Once that ends, you will start falling back down just as you would normally after Air Blocking an attack. However, the difference is that with Air Blocking, the initial portion of you falling back you are still in Block Stun. After the Advancing Guard Delay ends in the air, you can immediately perform any action.

Advanced Advancing Guard

Besides the obvious benefits, there are two additional benefits to Advancing Guard:
You can ignore chip damage from a limited number of hits during each Advancing Guard Delay (usually one)
You will not be affected by block stun during Advancing Guard Delay
The use for the first is fairly obvious: if you Advancing Guard at least every other hit of a multi-hitting attack, you can reduce your chip damage by half or possibly even more.
The second is mainly of use for punishing multi-hitting attacks. If you time your last Advancing Guard so that the Advancing Guard Delay ends just after the last hit is blocked, you will not be in block stun. Thus, you can make your frame advantage equal to the entire recovery of your opponent's move and punish appropriately.

A defending player can push block, or "Reaction Shot" as it is called in the Skullgirls tutorial mode, by pressing PP at any time during a block stun. The defensive technique pushes the attacking character away and prevents the recoverable portion of Chip Damage. If an attack from an Ensemble character gets push blocked, the Ensemble character gets the push back with no effect on the point character. Push blocking a projectile attack pushes the projectile's owner away.
Push blocking starts a 25f animation. When a single hit gets Push blocked, that single hit will always cause this minimum 25f block stun even when normal block stun is shorter than 25f. It is very possible for the defending player to actually lengthen block stun by Pushblocking at the very end of normal block stun or by Push blocking a move with a very short block stun. If any additional hits land during the Push block animation the additional hits deal the same softened chip damage, inflict only 50% of their normal block stun, and stop the push effect on the attacking character.
There are several strategies to efficiently use a push blocks:
Rapid fire push blocks to reduce chip damage from specials and Blockbusters with many rapid hits. This means push blocking the first hit then mashing PP until it's over.
Strategic push block against single hit pokes, projectiles, and specific hits in a block strings to create space. This is useful when the defending player doesn't want the attacking player anywhere near them for any reason.

First of all...don't call this maneuver a 'Tech Hit'. No one will know what you're talking about. Call it a Push Block, or Advancing Guard (AG for short). This is the most important defensive maneuver in the entire game, mostly due to the fact that you CAN learn to AG a single attack on reaction, much unlike a Guard Cancels.

(FEEL FREE TO SKIP THE NEXT FEW PARAGRAPHS IF YOU UNDERSTAND THE TECHNICAL DETAILS OF HOW PUSH BLOCKS WORK)

The way Push Blocks work is that you press attack buttons during blockstun. You need to press at least three buttons to even have a chance to Push Block, after which it happens between the 3rd and 6th input. The distance your opponent is pushed back depends on the strength of attack pressed when the Push Block is initiated. Thus, you really want to initiate Push Block with a Fierce or Roundhouse (Hp or Hk). Simultaneous inputs don't count, so you need to become proficient in pianoing the entire set of attack buttons.
This leads us to the proper way to AG on a stick: Lk, Lp, Mp, Hp, Hk (then Mk). There are two main reasons for this. The first and most obvious reason is that it makes sense when you think about it ergonomically. If you place your hand over the attack buttons of an arcade stick, they'll very close to the buttons listed respectively, with the thumb being over Lk and the pinky over Hk.
The second reason is more important: Probability. The first two inputs can't initiate an AG. From that point on, your chance to AG shoots up by 25% with each following input (thus, a 25% chance that the third input will AG, and a 100% chance that the sixth input will AG.
I don't like to think of those percentages, though, because they're misleading. Showing the probability is more important, because we need to take into account the possibility of a Push Block NOT occurring on certain inputs. By using math, we can more exactly determine the likelihood of which inputs will initiate a Push Block. I refuse to lay out the math here unless a bunch of readers demand to understand how probability works, but here's the results:
You have a 0/32 chance (0%) of AG'ing on the first and second inputs.
You have a 8/32 chance (25%) of AG'ing on the third input.
You have a 12/32 chance (37.5%) of AG'ing on the fourth input.
You have a 9/32 chance of (28.125%) AG'ing on the fifth input.
You have a 3/32 chance of (9.375%)AG'ing on the sixth input.
In other words, for the best results in AG'ing, use the Light Attacks first (you now have no chance to do a super wussy AG), and use Hard Attacks on the fourth and fifth input, because they have the highest probabilities of initiating the AG (you now have the best chance of doing the Totally Ultimate AG).

(TECHNICAL EXPLANATION OVER)

If your opponent can Push Block worth a crap, your rushdown will be that much harder to pull off. On the other hand, you should expect your opponent to be able to, or at least attempt it; if they're not, you're probably playing against another novice player (or an opponent who picked Anakaris because they hate themselves).
So, what can you do about AGs? Well...
1. Mix up your highs and lows.
No matter what, your opponent still has to block to AG. So, if your character has any sort of high/low mixup game to their normals, or even just a damn overhead - SOMETHING to make your opponent guess between high and low - use it to trip them up.
2. Empty Jumps.
Players who can AG will try to do so on reaction whenever they can, so if you mix up your jump-ins between jumping in with an attack, and a complete feint, you can then land right next to them and throw/command throw them. So, against AG-happy opponents, don't forget this trick!
3. Variable Blockstun Pressure.
This is the most important method of dealing with AGs. Using AG in close quarters can be dangerous. A lot of players will use it as they get up from a knockdown to stop someone's rush in its tracks, or they'll even just use it in close-quarters pressure situations. They will forego doing it on reaction, because there's plenty of benefits to simply TRYING to AG in close quarters:
If their opponent attacks, they'll AG, forcing the opponent back.
If their opponent tries to move in for a throw (or does anything to leave themselves open), The person AG'ing will throw out a light attack, which can be hit-confirmed into a chain. Even if they AG late, they'll probably still Tech the throw.
So, advanced players deal with this by using what's called 'Variable Blockstun Pressure'. What it means is, when you're pressuring an opponent in a close-quarters situation, instead of throwing our your (short, erratic) chains as quickly as possible, you'll want to leave a small window of time between one string and the next. It should be long enough that the person trying to AG will do it too early, throwing out an attack instead; but at the same time, it shouldn't be so long that they actually throw out the attack in time to hit you.