It is necessary that both hind legs move simultaneously and identically in both carrying and pushing manner while doing School jumps. Only few horses have two symmetrical hind legs. Most horses tend to put less weight on one hind leg and move the stronger hind leg further forward.
It is important for the rider to recognize this asymmetry from the beginning of the training. By training the horse straight, the weak hind leg can be made stronger so that the horse is later capable of using both hind legs equally.