Training a horse to respond to progressively lighter rein cues follows the same escalation ladder principle that applies to all pressure-and-release training, but applied specifically to rein aids: the rider consistently begins with the lightest possible version of the rein cue, gives the horse time to respond, and escalates only if necessary — immediately returning to the lightest version for the next repetition regardless of where the escalation reached on the previous one.
The common mistake that produces a heavy-mouthed horse is starting each rein request at the intensity level that reliably produces the response rather than at the lightest level possible. If a rider knows that a horse does not respond to light rein pressure until it becomes moderate pressure, and therefore always applies moderate pressure from the beginning, the horse remains a horse that requires moderate pressure because it has never been given the opportunity to learn that light pressure carries the same message.
The process of lightening a rein response works through progressive thinning of the cue: start with a feather-light contact, wait a full second for a response, reward any response — even a partial one — with an immediate and complete release and a moment of rest. Repeat, starting again at the lightest level. When the horse consistently responds at the lightest level, the response is established at that level and will remain there as long as the rider continues to start light.
For horses that have been trained to require strong rein pressure, the lightening process must also address the habit of ignoring light contact that the horse has built up over time. This requires patience — the horse will initially not respond to lighter cues because it has learned that light cues are not real requests. Consistent application of the principle across many sessions gradually shifts the horse's threshold, and most horses become significantly lighter in the hand within a few weeks of consistent work using this approach.