What is lateral flexion and why is it a building block for collection?

Lateral flexion is the horse's ability to bend its neck and body softly to either side in response to rein pressure, and it is one of the earliest and most important lessons a trainer establishes with a young or green horse. The reason it matters so much as a foundation for collection is that a horse that cannot yield laterally without resistance will never truly yield vertically, and vertical softness — the poll dropping, the jaw relaxing, the back lifting — is the physical expression of collection. The exercise itself is simple in concept. With the horse standing still, pick up one rein and bring it toward your hip or knee in a slow, steady motion. The horse should turn its head and neck in that direction and hold the flex without bracing against the rein. When the horse reaches the correct degree of flex and releases any tension in its jaw or poll, the rein is immediately released as a reward. The goal is not to achieve a dramatic bend but to achieve a soft, willing one — a horse that turns toward the pressure rather than leaning away from it. What lateral flexion teaches the horse is the fundamental equation of horsemanship: pressure means yield, release means rest. A horse that understands this equation in the lateral plane — turning away from rein pressure softly and without resistance — has already begun learning the same principle that will eventually allow it to yield vertically to bit pressure, step under with its hindquarters, and carry itself in a frame. Lateral flexion also relaxes a tense or fresh horse before more demanding work begins. A horse that is tight through its neck and poll will not be productive in a training session regardless of the skill of the rider. Two or three minutes of quiet lateral flexion exercises on each side allows the horse to mentally and physically soften before work begins, which makes everything that follows more effective.