The direction of force exerted by a spring.

The direction of force exerted by a spring.

The preceding figure shows a ball attached to a spring. You can see that if the spring isn’t stretched or compressed, it exerts no force on the ball. If you push the spring, however, it pushes back, and if you pull the spring, it pulls back.

Engagement—The act of flexing the hocks, stepping under the body, planting the hooves, and lifting. The forehand gradually becomes lighter as the hindquarters become stronger.
As a horse moves up through the levels, his frame—the shape of his topline and body changes. Becoming stronger and fitter, he can increase the degree of his hind-end engagement while you collect that engagement by, encouraging him to compress his body and lift his shoulders and withers more.
Shortening of your horse’s frame improve his concentration, increase fluidity of its movements, and build a stronger connection between you and your horse. The short (compress) frame slows your horse’s front end momentarily, while the back end continues at pace; the horse’s frame compacts, creating a topline arch in which his balance is on the haunches rather than the forelegs. This shift in balance “wakes” the horse and makes him more responsive to signals from you. Timing is crucial to coordinate your aids, so you may find it best to learn how to short your horse’s frame in stages.