You can juggle an opponent with a basic. After it, only specials and supers can juggle. Then you can only juggle with supers. VIDEO Street Fighter IV. Ultra convoluted. Every hit adds a point to a juggle counter. Every hit has its own juggle value. If the hit you perform has more juggle value than the opponent has juggle points accumulated, you will juggle succesfully. VIDEO Cvs2 and Street Fighter Alpha 2. Even more complicated than SF4. Took from http://sonichurricane.com/articles/capcom_vs_snk_combo_systems.txt
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Street Fighter EX series allows a timing window for juggles. If the opponent is in a state where he will land on his feet, you have a small timing window to juggle him. I don't know the exact number of frames. However, if your attack knocks the opponent down, he won't land on his feet anymore, not even with Standing LPs, until your combo drops or ends. Every attack you make will knock the opponent down again, giving a timing window of 27-28 frames (I don't know the exact number but I'm really sure that it doesn't exceeds 30). VIDEO Some plays have attacks that give a free juggle opportunity, knocking the opponent down, but without restricting the juggle window to 27 frames. Dhalsim's Yoga Blast, and Cracker Jack and Hokuto's super moves in the video above are among these free-juggle attacks. SF3: You have 6 juggle points. Attacks usually remove 1 juggle point. A few selected attacks remove more. Some like Ryu's Shinkuu Hadoken reset the points to 6, allowing 6 extra hits. When you have 0 juggle points available, the falling opponent is still vulnerable until he reaches the peak of his arc, when after it your combo is registered as finished. That's why in the movie below, some fighters juggle the opponent with 7 moves. VIDEO KOF. KOF experts can give more details than me. I don't know, for example, why in KOF'97, Terry can easily perform Power Charge > + B > + B > + B ... on the cornered opponent if he explods a Power gauge before, but can hardly do the same without exploding first. Or why in KOf'97 Joe can hit an opponent after a Screw Upper if the first hit whiffs, but can't if the first hurricane hits. Watch this video for extra discussion about KOF juggles (Andy's juggles worth special mentioning): VIDEO it looks like I missed the point of this thread. I mentioned how commercial games handle juggles, when actually it was supposed to be about which methods MUGEN creators use to do the same, and how effective they are.