Going to give it a go:
I think of it like a pause, a moment for the hindquarters of the horse to 'catch up' to the forehand.
To do this, you need to be aware of which leg is touching the ground at which time and cue at the appropriate time so that the leg on the ground has some leverage with which to execute the 'pause' because if you cue too early, you block the motion of the hind rather than using it as a base.
Because of the variety of situations you might use this in, the cues with the seat, legs and hands may differ.
ETA: Eliz, you and I posted at the same time!! Spyder, you posted while I was thinking ;)
All horses deserve, at least once in their lives, to be loved by a little girl.