The Back and the Pelvis
Commandant Jean Licart, "Basic Equitation".
It does not take a genius to recognize that a stiff body on top of a moving or bouncing body will cause jarring and banging. The human body enjoys natural flexibility and elasticity that allows it to act as a shock absorber and move in union with the horse. The motion of a horse in trot can throw about a rider who has locked his hip and back muscles, causing uncomfortable bumping against the saddle. By combining the flexibility concept with the principle of Jean Licart, one can reach the conclusion that total union with the horse can only be achieved through the separation of motion between the pelvis and the trunk.
To explain this further, a rider should remain upright with his shoulders square and his back straight, and allow his lower back and hips to move in collision with the horse, thereby separating the upper back from the pelvis, and achieving a still, quiet seat in the saddle.
The distribution of muscles in the lower back allow the pelvis to rotate and absorb the motion of the horse. By stretching and contracting these muscles, the pelvis can acquire the proper angles as the horse moves. When you flatten your back, you're stretching your back muscles and rotating the pelvis to a backward tilt. However, this should not be confused with the upright position in the saddle since the pelvis naturally stands at 0º. Also by exaggerating the curve in the lower back, you're contracting your muscles, and tilting the pelvis in a more forward position.
Sitting properly to the trot will give this concept a better meaning. As the horse steps underneath himself, you are pushed upwards (shock absorption in the lower back and pelvis will contract the muscles; then to absorb the downward drop, the muscles will stretch, allowing the pelvis to rotate backwards, thereby flattening the back.)
Although it may take time before sitting classically to the bouncy gaits is perfected, it does not take any strenuous effort. What needs to be learned is the relaxation in the pelvis and the lower back muscles. Once relaxation is achieved, the pelvis will move naturally and absorb the motion of the horse, letting the horse and rider appear as though they have become one body.
"If the rider is able to sit in balance with his seat bones well underneath him and his upper body in correct alignment over his hips, the pliancy of the spine in the loin area will allow him to remain as though glued to the saddle."
Sylvia Loch, "The Classical Seat"
The abdominal muscles are attached to the pelvis, therefore tensing these muscles will drive the pelvis out of its natural tilt. By keeping the abdominal muscles relaxed, the pelvis can move freely. With the horse stationary, the pelvis should be at its natural 0º tilt. In the forward, bouncy gaits, such as the medium trot, it is helpful to tilt the pelvis slightly backwards to aid in sitting deeply into the saddle. In collected gaits, some riders prefer to accent the curve in the small of their backs, rotating the pelvis slightly forward and shifting the weight from the seat bones to the fork.