From the very first time we get on a horse our legs move around. There are three main positions the legs wants to go to. Imagine for a moment being on a swing. Remember how you would use your legs. Bending your legs under your seat to entending them out in front of you. And how you could get the swing to go really high if you wanted to. If you simulate that same mechanic in your chair, you may feel how you rock up onto your pubic bone and back down to your seat bones. Now remember how you would try to get the swing to go with just your seat? How you would just bonce around and swivel but never get the swing to swing? If you use your seat to try to get your horse to move, the same basic idea happens, except that the horse will hollow its back. To ride is to be on a swing. The leg will move through the same swinging positions. It is over time that the rider learns to swing small enough that the leg appears motionless. Swing from the ball of foot connection to the sturrip with an unlocked ankle and feel the horse carry you happily along.