When the horse goes with a lowered back.
When a rider get up on a horse, the horse's natural reaction will be to lower the back. When the horse lowers its back, the vertebraes will come closer to each other. If this happens, the horse will not be able to work properly.
The horse will then find it difficult to step under himself. It may be difficult for the horse to bend. The fore and hind end do not work in harmony with each other, that is, the horse is struggling to keep the rhythm. The body's "shock absorbers" are put out of service and the horse's joints are put under heavy load, which can cause lameness, unnecessary wear and kissing spine. It goes without saying that this can not be particularly pleasant for the horse.