Throughout all of this training it is imperative the rider understand the half-halt and the halt. To explain them I offer this.
The halt is achieved by the deepening of the riders seat and stimulation of the horses hindquarters to reach forward and underneath his body, thus accepting a greater load of weight. The stimulation is the result of the riders leg pressure on both sides of the horses body causing the "lifting" of the spine to meet the rider's seat.
The rider will keep his upper body straight and push through the small of his back into passive and sustaining hands. They will accept any pull on the reins if necessary and remain unchanged. Once halted the rider allow a slight relaxation of the upper body and at the same time will advance the hands slightly to give the horse a period of relaxation. This relaxation is a vital part of the training process because without them nervousness and restlessness will set in. Any roughness or excessive action on the part of the rider will cause the failure of a soft and fluid stop.
In the half halts the method is the same with the sole difference being that the hands will allow the motion to continue. In this case the driving controls will outweigh the restraining ones.
As far as half-halts it is the one subject that seems to create endless discussions and usually ending in no one being any the wiser than before. When you go to forums dealing specifically with dressage and you still see the endless pages of discussions by well known trainers and no real resolution then we are in good company.
Different trainers create descriptions of what it is and what it does to the point that we appear to be entering a country with a different language. Combine this with its use that will vary as the horse progress causes its interpretation to be based on one horses reaction. However when applied equally to another the cause and effect can differ greatly.
On the basic level the half-halt is used to signal the horse that a decrease in rate is being asked for. It could also be a signal to improve its balance or lighten the horses weight on the reins.
On the highest level the half halt becomes the merest whisper of controls in that it is created with the seat and hinted at with the reins.
My dressage instructor explained that the half halt is intended to get the horses attention. "Change coming!" Whether you then change a gait (up or down transition), collect the horse when it gets chargy or strung out, etc.
If you think about it a person half halts when approaching a barrel turn. You sit deep and the horse prepares for the turn.