What do Anderson and Parelli say about horses that open their mouths or gape when the bit is used?

A horse that opens its mouth or gapes when rein pressure is applied is communicating discomfort, and both Clinton Anderson and Pat Parelli identify mouth opening as a diagnostic signal that should be investigated before it is corrected. Anderson's diagnostic approach begins with bit fit and dental health. A horse that gapes may have a bit that is too wide or too narrow, a port that contacts the palate, a mouthpiece that contacts the teeth when rotated, or dental problems — sharp points, wolf teeth, or sores — that make any bit contact painful. He recommends having the horse's teeth floated and a veterinary assessment of the mouth before assuming the gaping is behavioral, because treating a pain response as a training problem with more bit pressure typically makes gaping worse and more entrenched. If the bit fits correctly and dental health is confirmed, Anderson addresses gaping as a training issue — specifically as a horse that is bracing against bit pressure rather than yielding to it. His correction is returning to snaffle work and rebuilding the softness to bit pressure through systematic give-to-pressure exercises until the horse yields without bracing, then reintroducing the curb bit with significantly lighter contact than was previously used. Parelli frames mouth opening as a Right Brain Introvert or Left Brain Extrovert response depending on the horse's underlying temperament, but his practical prescription is similar: investigate the physical cause first, then address the training if no physical cause is found. He also notes that horses ridden with fixed, tight contact are more likely to develop gaping than horses ridden with a following, elastic contact, because a hand that never releases gives the horse no moment when yielding produces relief — so the horse stops looking for relief and starts looking for escape through opening the mouth.