There are many techniques explaining how to stop the horse. But none of them will work if you are tensed when you do them.
Your tension makes it impossible for the horse to relax, and to allow his back to welcome your seat. Your tension makes it impossible for the horse to hear anything of what you want to say.
When I teach my students, the very first step towards both moving forward and stopping is to remove all the tensions from their bodies and minds that may block the movement forward and possibility of a relaxed halt.
Dressage square halt is a wonderful experience both for a horse and a rider. I see that many people focus prematurely on the SQUARE quality of a halt. They halt the horse, and they correct the legs.
What horses taught me, is that a relaxed horse that welcomes a relaxed rider in his back WILL ALWAYS DO A SQUARE HALT.
If you have problems with stopping your horse,
If your horse is not stopping in a nice, square position,
If your horse is stopping on the forehand,
It only means that you are tensed!
Because of your tension, your horse travels out of balance, crooked and gives you a feeling of the late responses.
Experiencing relaxed, square halts through expanding rider’s awareness of his body should be the very first step before any advanced riding techniques can be applied.