"Over time the halt aid develops as a nuance of the collecting aids. When the horse is uphill and collected in the walk, the seat asks his back feet to stop up underneath him, poised and loaded, ready to proceed in walk, trot, canter or rein back.
"That is why the halt must always remain a forward transition, so the horse is ready and waiting to proceed into the next phase.
When the horse is halted, he must sit a little and bend his hind legs to take weight in them. This strengthens the horse’s carrying strength. When the rider then asks him to move forward out of the halt, the horse has to push off with his hind legs, which strengthens the pushing power. The half-halt is just a mini-halt that momentarily checks the forward movement and shifts weight back. The horse is then driven forward to increase the spring in the steps. The rider should always strive to balance the forehand and the hindquarters. This will be discussed more throughly later in this article.
--Cynthia Hodges, MA