Aids are a method of communication between horse and rider. Of course, clear communication is important for a healthy and productive relationship. Both the horse and rider must understand certain cues and how they work. The cues come from the rider's hands, voice, legs, and seat. Thru this method of communication the rider can let the horse know what is requested.
When the aids are thoroughly understood by both horse and rider and used with precision, they can produce a harmonious relationship. It will appear to anyone watching that the horse and rider can read each other's mind as they are very much in tune with one another.
In basic training, a horse is taught to move away from pressure. Under saddle, this same training is in effect by weight in the seat, a leg on or off, or a slight tension in the rein. Pre-cues are an important part of the aids. Consistancy in pre-cues helps the horse learn the aids faster.
A horse can learn certain words under the premise of pre-cues. If the word "whoa" is used everytime before he feels the shift in weight for a stop, he will learn to anticipate and stop on the verbalization of the "whoa".
Use your hands lightly. True communication between horse and rider is not thru the hands. Controlling a horse truly is mind to mind.
Proper use of the aids can keep a horse and rider happy.