I tend to do it on impact. Not after impact, but press B so that the frame you hit on is the frame B is pressed. Note that the red/orange hit sprites appear in front of the opponent, not in the middle of them, so press B when Taokaka should be just in front of the opponent. That's how it works for me. Basically, you have to do ~B regardless of whether or not it hits (since this is a combo, you should essentially always hit, so no wories there) because your brain has to tell your finger to press B before you actually hit.