Execution, mix up game, frame traps, block strings, jump ins, wakeup game. All of these, they would be nothing without momentum. Momentum forces your opponent to block, lets you jump without getting hit, let's you do everything you want to your opponent without fear of getting hit. This is where mastery of footsies comes in. You can't pressure your opponent until you can safely get close to them. So here's a few things you'll need to gain the momentum.
Defense - The key to staying alive. You want to do what it takes to gain momentum, you definitely don't want to give it to your opponent. This includes anti airs and keepout attacks. Moves that keep your opponent from getting in on you. Some characters can get momentum off these defensive attacks. Defense also requires you to analyze your opponent's offensive patterns to break his momentum and start your footsies again.
Keepout Attacks - Attacks that are used to keep your opponent out of range. An advancing opponent will not be holding back to block. For example, Ryu's cr.mk xx fireball. It's nice, you get a small chunk of damage out of your opponent if he tries to advance. Fullscreen fireballs cause pushback and forces your opponent to block or find an alternative way around them.
Offense - This is where it gets pretty tricky. What you have to do is, read your opponent's defensive patterns. Try to figure out what he won't be expecting, what he can't counter and take advantage of it. Some people can't handle certain jump distances, some struggle against slide attacks. Reading your opponent and taking advantage of the openings is the key to gaining momentum.
Once you gain your momentum, you can have your way with your opponent until he breaks out or...gets stunned. Good luck with this guys, hope I helped a bit.
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