The circle of aids is an important concept in the rider’s training. It refers to the flow of energy within the horse and to the way the rider’s aids stimulate and channel this flow.
The circle of aids typically begins with a driving calf aid that brings the horse’s hind legs closer to the center of gravity and into the sphere of influence of the seat. If the hind legs are too far out behind, they are out of reach for the seat and the horse’s back drops. Trying to apply any seat aid at that moment will make the situation worse. It will cause the back to drop - and hurt - even more, and the hind legs will then actually be prevented from stepping under by the weight aid.