This came in as a question: What is the timing of the whip in trot?
Answer: As the horse begins to pick up his hind leg on the whip side.
I have a rather long post on the use of the whip, done in the depths of winter last year here:
The timing is really easy, but takes a little practice.
You are holding your whip correctly over your thigh, the heel of your hand slightly pressing it downward.
You can feel your leg move in your hand as the whip is connected to both.
As the horse begins to step forward with the hind leg (the one on your whip side) you will feel your hip and thigh dropped a bit. Your hip, like the horse’s hip, are no longer supported. And, as the horse bends his hind leg folding it to swing through, that side drops.)
If you follow your thigh around and tic with the whip at this point the rhythm of that hind leg will be quickened. And assuming you do not let the horse sprawl in front or rush off, then the diagonal pairing will be improved.
Oddly, though you should be able to do this on both sides, it improves both hind legs as long as you make sure the horse travels straight. This is true in piaffe as well and why it is important to make sure the horse travels straight.