It's a dressage term, so maybe that accounts for why you have not heard it before.
I think it means that you ask the horse to make some shifts in it's way of carrying itself, as if he were being asked to halt, but you do not halt the horse. instead, you take the hrose, who has rebalanced himself ever so slightly as if about to halt, and you ask for a change of some kind, such as go faster, come down a gait, step under more, bend more.
So, it' used as a way to keep the horse ready and responsive to the next cue the rider will give.
I will let others explain how it' done.