netg, how you do leg yield is how I do it too, but the question wasn’t “how do you ask for leg yield?”, it was “why not use the whip when the hoof is in the air?” which is, in my mind, a separate issue. The first two answers I gave were explanations I’d heard/read/learned somehow and at some point for exactly the latter question. I don’t think they’re quite right though; I think it’s more about coordinating the whip to the leg aid. Across the board, I think the more-or-less right answer to “what is the correct timing for the use of the whip in training” is “when it maximally supports/enforces other aids.” It’s not about micromanaging at all.
It took me a while to formulate the rest of my response, actually, because I, like you, had to think through what the timing of my leg aids is relative to the footfalls, as I don’t think about it while I’m doing it. I probably do start putting my leg on in the loading phase, tbh, and then it just gets stronger through the departing phase. On the horse, it feels more like a whole body exercise (am I straight, are my legs in the right place, am I balanced, is the horse forward enough, etc., and then I shift my weight and we just go), which makes being effective way simpler than worrying about specific timing of various aspects of it separately. So I totally agree with you in that regard. Then again, it does take me at least 3 strides to make any significant changes to whatever I’m doing, so I probably should pay a little more attention to the timing!