Take any advice from me with a grain (or ten) of salt, since footsies is by far my weak point. The ‘punishing whiffs’ part of footsies I’m decent at, though.
As cliched as it sounds, practice makes perfect. I used to think that punishing Sagat’s fierces was nearly impossible, now I think it’s cake. Although it would seem that you need incredibly fast reflexes to punish whiffs, people with slow reflexes like me can still do it if they concentrate on it enough. It’s not even as difficult as rolling through fireball thrown up close.
The way I punish whiffs (which probably isn’t the best way), is I jiggle around at about the range of the move my opponent is most likely to press. In Sagat’s case, hang around at the tip of the range of his low fierce/forward. Sure, he can reach you from there with standing forward but it’s not that big of a deal. Most characters can punish Sagat after blocking/getting hit by Sagat’s standing forward anyway. So, just get around the tip of Sagat’s fierce range and jiggle back and forth like you see the other top players doing. For some odd reason even if Sagat hits fierce while you’re in range you end up blocking it 95% of the time without even meaning to. While jiggling there just watch your opponent’s sprite. If it does some kind of attack just hit your “whiff punisher” button regardless of what happens; if your opponent’s attack whiffed then you get a free hit. If you’re within the opponent’s range and you block the move and still hit your whiff punisher button nothing will come out since you’ll be in the middle of block stun. Simple enough, right?
The two main weaknesses to this strategy is that you can be baited by an opponent whiffing jabs and shorts, and that while jiggling around 99% of players will not block low at all. The latter is hard to fix; it’s been Ricky’s main weakness for a while now. However, most characters don’t have low moves that reach far enough to take advantage of this, and even if they do those moves tend to be very risky to do. Examples of these moves would be Iori’s low roundhouse, Hibiki’s qcf+fierce, Bison’s and Blanka’s slides, etc. The former weakness can be effectively patched up by doing what I call “low risk whiff punishers”.
Most decent characters have two good whiff punishers; one with high risk/reward and one low risk/reward. For example, Hibiki. Hibiki can punish whiffs very easily with her low fierce, but if she gets baited by like a whiff jab or something and the opponent jumps or rolls through it, she’s dead. To help lessen the risk, sometimes the Hibiki user should punish whiffs with low/standing strong instead. While it’ll take a million strongs to kill somebody, the strong is 90% safe to rolls and jumps if it is baited. When should you use the high risk moves? When you are 90% sure your move is going to hit, like after your opponent whiffs a fierce or roundhouse.