One of the major issues I observe in riders at the sitting trot is the ability to maintain the connection of the sit bones through the entire cycle of the gait. When we loose the rhythm of the cycle of movement the body has to tighten to hold on and in a preparatory reaction to the possibility of hitting the ground. This happens whether we are asking for it or not. The movement in the rider's body looks more like a rising or posting trot than it does the sitting trot.
The movement of the body at the sitting trot should be exactly like it is at the walk. The only difference being the power and the speed in which the body is moving with the horse. The movement of the pelvis, femur bones and lower lumbar mirror that of the rider walking effectively on the ground.