Rein aids for transitions between gaits work in coordination with seat, weight, and leg aids to produce smooth, balanced, willing gait changes — and the quality of a horse's transitions is one of the most reliable indicators of the quality of its overall training. A horse that transitions promptly, smoothly, and in balance, without throwing its head, rushing, or falling on its forehand, has been trained with clear, coordinated aids and has learned to respond to those aids without resistance.
For downward transitions — from canter to trot, trot to walk, walk to halt — the rein aid is applied as a brief closing of the fingers that says organize and slow, supported by the seat closing and the back muscles engaging. The rein component alone, without the seat and back, tends to produce a head-raising, resistant response rather than an organized downward transition. The leg remains active through the transition to maintain the horse's hindquarter engagement — a downward transition produced by rein pulling alone removes hindquarter engagement and produces a horse that falls onto its forehand in the new gait.
For upward transitions — from halt to walk, walk to trot, trot to canter — the rein aid softens and gives at the moment of the transition, allowing the horse to step forward into the new gait without restriction. The giving hand at the moment of an upward transition is as important as the asking leg — a rider who gives with the leg but holds with the hand blocks the horse forward and produces a resistant, reluctant upward transition.
In Western performance, the transition aids are the same in principle but more precisely defined by the specific discipline demands. A reining horse's transition to canter must be prompt, smooth, and on the correct lead — the rein aid coordinates with the leg aid to specify not just canter but left lead or right lead by positioning the horse's flexion and shoulder in the direction of the requested lead.