It depends on who you ask.
In general, people think it is a use of the reins, but this is more like what used to be called a 'Check' and is used to slow down the horse by having him slow down his strides. A 'check' is all most people use, they use it to stop, to slow down, it is a taking action, usually on both of the reins.
In dressage, it is an almost-simultaneous use of the rein, seat and leg. At some point in the rider's education, he gets to the point where he doesn't even think about it, it's automatic.
Most instructors teach it as if it is 'half of a halt', and it is more of a fancified 'check' than a half halt; it is still used to slow down, to make a downward transition, to slow the rhythm. You can see the horse's hind legs pause in the air or slow down.
Even more confusing, the first phases of teaching it, do involve, even from the best instructors, slowing the horse down. Most people never go beyond that, though. Once they can slow their horse down, they're satisfied.
But that is not its real purpose. The initial stages are only to get the horse to put his hind legs more under him, it's kind of that most rudimentary 'explanation phase' of teaching something.
Later on, the rider could half halt a dozen times in a few strides of extended canter or extended trot, and the horse would not slow down one bit. All it would do is engage (bend the joints of) the hind legs more, which allows his hind legs to work more powerfully - thrust more or carry more.
The rider more sets the rhythm or stops or slows the horse, with his body. The half halt only puts the hindquarters right, it doesn't slow or stop the horse.
When a half halt is done correctly you can VISIBLY see that the hind leg bends more. The joints of the hind quarter and hind leg bend more. It is obvious, visible in effect.
There is a whole lot that has to happen to make a half halt possible. There needs to be a lot of forward activity, and contact with the reins.
The horse has to be very comfortable with moving forward, and with connecting with the rider's hand. If he is afraid of the bit, if he is taught to 'set his head', half halts will just turn into a mess.
Dressage is a conversation, and the words are all made up of half halts.
Last edited by slc; 02-04-2011 at 10:06 AM.