Biomechanics of the spiral seat

Only the rider in the top and bottom images may use the spiral seat. The other two riders have limited the possibilities of "going with" or following the movements of the horse by having the upper and lower body tensed out of position, breaking or rearranging the contact. Click on images for large version.
Level balance, rider relaxed into spiral seat with elastic connection stirrups (foot placed so joint of big toe is placed to maximize stable rider posture.
On forehand, rider behind vertical (in the manner of women) with too much weight on the seat. The solution to this problem is to balance more on stirrups so ankles, knees and hips can flex to absorb motion of horse. For people with bad backs, this position might be problematic. The driving seat affects the integrated functioning of back muscles, so the horse is said to "go in two pieces" or "lion in front and mouse behind."
Horse tensed in topline and bottome line, with rider gripping and perched on thighs. The solution to this problem is to balance more on stirrups so ankles, knees and hips can flex to absorb motion of horse. Gripping with thighs locks the pelvis in position, preventing use of the spiral seat to follow the coordination of each gait.
The "circle" or "loop of aids" and the independent seat are important for gymnastic development of a horse. This seat rides a horse continually, elastically "toward the bridle" from leg and pelvis. Muscles in pink are primarily influenced by seat while those in blue are strongly influenced by leg aids. Division of labor (dancer's isolation of muscle actions) in a rider's body takes practice: relaxation and patience are important. The seat is very complex, as a diagram indicates in the rider and horse seen from behind.