Have a detailed read, it explains the spiral seat (depicted with the grey rider, not the skeleton) http://www.dingosbreakfastclub.net/DingosBreakfastClub/BioMech/BioMechRide3.html
On this image: Horse tensed in topline and bottom 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.
More on how to properly work your ankles, knees, and hips together rather than bracing and being tense, which seen here makes your horse tense.
The paw prints and hoof prints of a few meddlesome animals have been preserved for posterity on ancient Roman tiles recently discovered by archeologists in England.
"They are beautiful finds, as they represent a snapshot, a single moment in history," said Nick…