Yielding to pressure on the ground and yielding to leg pressure under saddle are the same principle applied from two different positions, and the connection between them is so direct that groundwork which establishes clear yielding on the ground dramatically reduces the time and confusion involved in establishing the same responses from the saddle. A horse that has learned to move away from light pressure on its barrel from the ground already has the foundational understanding of what leg pressure means — it means move away from the pressure, in the direction indicated.
The translation from ground to saddle happens most smoothly when the groundwork specifically prepares the horse for the spatial relationship of a rider's leg. Teaching the horse to move sideways from finger pressure, dressage whip pressure, or a flag applied to its barrel in the position where a leg would hang gives the horse a frame of reference for what the pressure means before the additional complexity of a rider's weight is added. When the rider's leg is applied in the same position and with the same intent, the horse recognizes the pattern.
Leg yielding, turn on the haunches, turn on the forehand, side-passing, and shoulder-in under saddle are all specific applications of the move-away-from-pressure principle to different combinations of rein and leg. A horse that truly understands yielding to pressure can learn each of these responses relatively quickly because it only needs to learn which specific body part to move and in which direction — it already knows the fundamental concept that pressure is a signal and that moving away from it produces release.
Al Dunning's work on Western performance horses consistently demonstrates this connection — the advanced lateral maneuvers that cutting horses and reiners execute with apparent effortlessness are built on a yielding foundation that was established in the horse's early groundwork. The sophistication of the finished maneuver is directly proportional to the clarity and depth of the yielding education underneath it.