Give and take with your reins
Half halts can be used in many different ways. A half halt is as exactly what it sounds like, a half of a halt. Or a light wiggle of your fingers on the reins. Half halts can be used to slow down a horse, which is much more affective than just pulling on the reins to get your horse to stop or slowdown. Half halts can be used to tell your horse that something is about to happen, such as a transition or regathering before or after a jump. Half halts can also be used to soften a horses mouth and to get them to relax. The half halt can be used in many different ways, if you look into it more there are probably even more uses for it than the few examples I gave.
I agree with Rile on how to do the half halt. The idea of a half halt is that your horse will collect itself and engage its hind end. The horse should "suck" it's shoulders up and get more suspention in the front while slowing down slightly.
This actually has absolutely NOTHING to do with your reins. You have 4 muscles in your back. 2 that criss cross and 2 that run up and down parallel to eachother.
To perform a half halt you are to squeeze the 2 parallel muscles and push that strength into your horse.
To practice this you can sit on a folding chair on the very very VERY edge of your seat bone. Basically your whole body will be hanging off the chair. And WITH OUT moving back and WITH OUT leaning forward
You want to sit up straight and squeeze the back muscles to try to drive and lift up the back legs of the chair.
Hope this helps!!
It slows down the horse without making them change gaits. If you're trotting fast and you want a slower, collected trot, sqeeze the reins for a second. Alternate reins if they're not cooperating. As heather said, it also gathers the horse.
A half halt is something to gather the horse. For example when your turning and you want to gather your horse and get them collected for the fence you would sit deep and slightly pull back on the reins towards your hips then the second you feel them give and collect them self you release immediately