Most of them require you to do it early on in their attack animation for whatever move you were pulling off. Especially for fierce moves that do multiple hits. I don't think you can cancel into anything else if you've hit twice with a move. Fast indeed. You pretty much have to motion them quickly and consecutively. Usually just praying the first move connected, 'cause once you start up your super or ultra and they're blocking, its too late and you're pretty screwed