In dressage, the rider uses his weight, legs and seat to influence the horse. These are known as the “aids”. In order to use the aids correctly, the rider’s body must be aligned and balanced. From a side view, the ear, shoulder, hip and heel of the rider should be in a straight line at the halt.
From the back, the rider should sit evenly on both seat bones and the length of the stirrups should be the same.
By shifting his hips and his weight, the rider can ask the horse to move in different directions and step into different gaits. In simplest terms, the rider should have his hips do what he wants the horse’s hips to do and his shoulders do what he wants the horse’s shoulders to do.
For more specific information about the rider seat and aids, you might consider reading the following books available in the USDF bookstore: