Animation number: 5950The standard cheese kill animation. Usually triggered by being KO'd by block damage, but certain MUGEN characters also trigger this animation. However, if you are coding an attack that causes a collapse, I would recommend checking if 5955 exists for the opponent, as some characters have different animations for collapse and cheese kill. There is neither a fixed time for the animation nor a sprite requirement; however, on the last frame, do NOT set the time to be -1. Instead, put a loop that final frame, otherwise this will cause problems with special attacks that put you in this animation, and the match will not end if you're cheese killed.
Notable examples: P.o.t.S.'s Shin Gouki (Misogi), Vans's Oswald (2, his throw), and Zero-Sennin's Hibiki (Divine Spirit of Victory)
Animation number: 5955The standard collapse animation. (Usually) exactly the same as 5950 (some characters, such as Rolento, have differing collapse and cheese kill animations); this animation, unlike 5950, has CLSNs, so the collapsing player can be hit as they're falling to the ground. If the opponent doesn't have this animation, you should avoid using 5950, because there is no guarantee that any given opponent's 5950 would contain any hitboxes. In that case, you'll have to use a custom animation made up of required sprites. Better safe than sorry!
Notable examples: none, because I just made this up. Start using it!
A note about characters who collapse and reverse directions: Certain characters, such as Raiden and Zangief in CvS, end up facing the opposite direction from the one they were facing at the end of their collapse animation. This poses certain problems, but they are easily fixable: if left normally, the character in question will change back to their original orientation when their get up from lie down animation plays, producing a very jarring effect. To avoid this, horizontally flip the last frame of 5950 in your .air file, and insert this into your character's -2 state:
[State -2, Switch directions after collapse]
type = Turn
trigger1 = Anim = 5950
trigger1 = AnimElemTime(AnimElemNo(0) + 1) = 0
This will cause your character to seamlessly change directions, producing a smooth flow from 5950 to the get up animation.
A note about characters who collapse face-down: in certain games, such as later KOFs, the collapse animation sends the character into a face-down lie down state. If you happen to be coding a move that sends an opponent into a custom state that uses a collapse animation, you should check to make sure if the character has a face-down lie down animation, in which case the collapse animation will almost certainly have that character fall face-down. This piece of code will detect whether that is the case and change to the proper lie down animation if needed.
[State X, ChangeAnim to face-down lie down]
type = ChangeAnim
trigger1 = AnimTime = 0
trigger2 = AnimElemTime(AnimElemNo(0) + 1) = 0
value = ifElse((Anim = 5950 && SelfAnimExist(5112) || Anim = 5400 && SelfAnimExist(5112) || Anim = 6010 && SelfAnimExist(5112) || Anim =5800 && SelfAnimExist(5112) || Anim = 5130 && SelfAnimExist(5112)), 5112, 5110)
persistent = 0
Simply add to the state where the collapse animation plays, and you should be fine. Note that if you keep the opponent in a custom state as they're lying on the floor, and then return them not to their own lie down on floor state but their get up from lie down state, there is no need to include this. But there will be certain attacks that require P2 to be sent into their own lie down state when they hit the floor, so this is needed.
To replicate KOF hard knockdown behavior, the following animations are required:
Animation number: 5420The hitshake frame for being knocked down while standing. Consists of one frame (some characters use one of their standard get hit sprites, others have unique frames), with a time of 1.
Animation number: 5421The hitshake frame for being knocked down while crouching. Consists of one frame (one of their get up from face down liedown sprite, for all characters, but it's possible someone might have a unique frame), with a time of 1.
Animation number: 5422The character, crashing into the ground. Consists of one frame (some characters use one of their standard get hit sprites, others have unique frames). Timing is generally 4 ticks, but could be changed, if you want, though the code behind this will automatically proceed to the next animation in sequence after 12 ticks.
Animation number: 5423The character falling to the ground after having their face slammed into it. Some characters have only one frame for this animation; others have multiple, and will either use standard get hit sprites, or have unique frames. Timing is variable (but usually around 7-10 ticks), though the code behind this will automatically proceed to the next animation in sequence after 12 ticks.
Animation number: 5424The character rebounding off the ground. Some characters have only one frame for this animation; others have multiple, and will either use standard get hit sprites, or have unique frames. No timing requirements.
Animation number: 5430Aerial knockdowns require only this animation, the character rebounding off the ground after falling from the air. Generally consists of various gethit sprites flipped and realigned, though some characters have unique frames for this.
Notable examples: KoopaKoot's Kyo, Vans's Shingo, CrazyKoopa+Jmorphman's Ryo (this uses a variant in which 5423 is skipped for a more Capcom-ish feeling, and 5430 is not used at all)

Animation number: 5940The character escapes from a throw before it can be executed, pushing away the opponent. No set time length or sprite requirement, though the last frame should have a time of -1. For characters in Capcom games, this animation typically uses the same sprites as their guard crush animation; however, for KOF characters, the animation is often one of their attack animations.
Notable examples: Many characters, including ones by PotS', Vans', H'', and more

Animation number: 5945The counterpart to 5940, this is the animation that plays for the player whose throw was escaped. No set time length or sprite requirement, though the last frame should have a time of -1. For characters in Capcom games, this animation, much like 5940, typically uses the same sprites as their guard crush animation. For KOF characters, the animation is sometimes (but not always) composed of various get hit sprites; other times it uses unique sprites.
Notable examples: Many characters, including ones by PotS', Vans', H'', and more
what have I gotten myself into???