Using rein aids to develop lateral flexion — the horse's ability to bend its neck and body softly in either direction — is one of the foundational applications of rein communication and one of the exercises that most directly develops the suppleness and responsiveness that all subsequent work depends on. A horse that cannot flex softly in both directions, that is stiff to one side or braces against lateral rein contact, will show those limitations in every circle, turn, and lateral movement it is asked to perform.
Lateral flexion from a standstill is the most basic introduction: the rider takes up a feel on the inside rein and asks the horse's nose to come toward the rider's knee, releasing completely and immediately when the horse's jaw softens and the nose moves in the desired direction. The key is that the release comes for softness — for the giving of the jaw and the releasing of the poll tension — not merely for the position of the nose. A horse that bends its neck while staying tight through the jaw and poll has not truly flexed.
From a moving horse, lateral flexion is asked in the same way but at the trot and canter, where the horse must maintain forward impulsion while bending — which is a more sophisticated demand than flexing from a halt. The rider asks for inside flexion with the inside rein, maintains forward energy with the inside leg, and regulates pace and bend with the outside rein. The horse that can flex softly at all three gaits without losing rhythm or impulsion has developed genuine lateral suppleness.
Jane Savoie's systematic approach to developing lateral flexion specifically addresses the sequence of introducing flexion — starting from halt, then walk, then trot — and the importance of equal development of both sides, because most horses have a naturally stiffer side that requires more consistent work to develop suppleness equivalent to the naturally softer side.