Spoilered in case it counts as a derail:
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Just to give you an idea on what a hitstun/blockstun is, it's a small "stun" after a hit/block for a brief period of time.. Like let's say I use "hp" to hit my opponent, and he's in his "hurt" animation. If that opponent is in his "hurt" animation for a long time, I can use that advantage to continue a combo via a link, or chain.
Blockstuns are sort of the same thing, except it's how much the opponent is stunned via a block. If that stun goes on for too long, I can use that advantage to either mix-up, or throw.
Hope this helps out.