Teaching a horse to yield its jaw — to release tension through the mouth, lower jaw, and poll in response to bit contact — is one of the most delicate and important applications of the yielding principle because the mouth is where the majority of rider communication is transmitted and where the consequences of training errors are most immediately visible. A horse with a tight, resistant jaw is a horse that cannot fully receive rein communication, and no amount of additional rein pressure will improve the situation — it will only create more tension.
Jaw yielding is introduced before any rein work begins by teaching the horse to release tension through the jaw in response to gentle lateral flexion of the head. Standing at the horse's shoulder, the handler applies very light lateral pressure on the halter to bring the nose toward the hip, and waits — not pulling, just maintaining a steady light contact. The horse will typically brace initially, holding the jaw tight and resisting the lateral pressure. The handler waits for the slightest release of jaw tension — often signaled by a chewing or swallowing motion — and releases completely at that moment.
Progressing from halter to bridle, the same exercise is done with the rein — standing at the horse's side, taking up a light inside rein contact, and waiting for the horse to yield through the jaw and poll rather than brace against the contact. The release when the horse softens is immediate and complete. Over many repetitions the horse learns that softening through the jaw produces the release of the rein contact, and it begins to offer that softening more quickly and from lighter contact.
Ken McNabb's work emphasizes that jaw tension is almost always connected to poll tension and that the two must be addressed together — a horse that has learned to yield its poll and drop its head will typically release its jaw at the same time, because the two anatomical areas are connected through the same muscle chains. Exercises that address poll yielding and jaw yielding simultaneously are therefore more efficient than those that target one area while ignoring the other.