"Now being upon the haunches (as he necessarily must be in this case) is it impossible but he must be light in hand, because no horse can be rightly upon his haunches without being so"

This quote by William Cavendish is describing the significance of light hands when riding a horse. When a horse is on its haunches, it remains in a position where the rider must have a light hand in order to keep balance and control. If the rider has a heavy hand, the horse will not have the ability to stay on its haunches and will be not able to carry out the preferred movements. For that reason, it is necessary for the rider to have a light hand when riding a horse on its haunches. This quote stresses the value of having a light hand when riding a horse, as it is difficult for a horse to remain on its haunches without it. A light hand is necessary for the horse to be able to perform the desired motions and for the rider to maintain balance and control.