As a beginner, you will have likely learned that to use your hands, you need to pull on the reins, and to use your leg, you kick or squeeze the horse. While this is fine for beginners, as you advance in your skills, you will find that you need to use more subtle aids to get the results you need.
To understand how a horse reacts to the aids, you need to realize that a horse’s reactions are all based on a reaction to pressure. When a horse feels pressure, he will seek to release the pressure, usually by moving away from it. When training a horse, a horse learns which types of actions result in the pressure going away.
When you use your leg, the horse moves away from the pressure that you apply. If you sit on your horse and squeeze with one leg while maintaining a loose contact with your reins and other leg, your horse will seek to escape that pressure by moving sideways and forward. This is because he has been trained to move away from your legs by moving forward. The uneven pressure will encourage him to move sideways in addition to forwards.
To properly use your leg, you need to maintain leg contact on your horse at all times. This will keep the communication line open so that he will be able to respond quickly to your aids. A maintained contact also keeps your horse steady so that he will respond accurately to your aids.
If you squeeze with both legs, your horse will move forward. This is a conditioned training that he has received from when he was started under saddle. If you squeeze only one leg, your horse will move away from that contact by bending his body.
A leg applied at the girth will signal to your horse that you want him to move his front end away from the pressure. A leg applied behind the girth will signal that you want him to move his hindquarters away from the pressure.
Of course, you also need to use your hands to help your horse understand your aids. By maintaining a contact on the bit, you will be able to keep your horse supple and attentive to your aids. A well trained horse will not only accept the contact, but will also seek that contact.
Many rein aids are given, not by pulling on the reins, but by maintaining the contact. For example, to ask a horse to halt, a rider maintains contact, restricting the movement of the hand so that it no longer moves with the horse. The horse, feeling this restriction, will stop moving forward into the contact, thus coming to a halt. It is almost like setting up a wall in front of your horse, stopping his motion without pulling him back.
If you maintain your leg into the hand, your horse will not only come to a halt, but will remain engaged, using his hindquarters as he comes to the halt, rather than stopping by falling onto the forehand, with his hind legs strung out behind.
Because a well trained horse will seek contact, it is possible to turn a horse with nothing more than a slight release of the outside contact and pressure from the inside leg to tell the horse to turn. Correct application of the rein and leg aids can ask the horse to perform any number of movements, all based on his desire to release excess pressure, and maintain contact.
It is a joy to watch a well trained horse and experienced rider work together. With nothing more than the slightest aids, the horse and rider can perform in such unison that a casual observer would not recognize that the aids are there at all.