when you stun someone for a certain amount of health (indicated by a whit flashing bar on their health bar. shows how long they are stunned before they get knocked down) they will enter a red "critical stun" period for one or two hits. each character has a move that, when successfully hits, initiates the opponent into a Critical Burst state. (it's very obvious when you get this off successfully. a loud SMASH sound happens.) after then you can do anything the the opponent (don't know about grabs) because they cannot block nor counter in the state. once you make the opponent enter this state you can either launch them (they will be launched higher then usual. like a hi counter launch.) or you can use your power blow if your health is below half.
To add on what Vincent said you can see what your critical burst and power blow command is when you go over your character so you don't have to figure out what the command is.
