Using rein aids to develop self-carriage — the state in which the horse carries itself in a balanced, uphill posture without leaning on or requiring constant support from the rider's hand — is the goal toward which all rein training progresses. Self-carriage means the horse has internalized the balance that the rider's aids have been developing and can maintain it independently between those aids rather than depending on the contact for support.
The rein aid most directly associated with developing self-carriage is the give — the deliberate softening or releasing of the rein contact — which tests and trains the horse's ability to carry itself without support. A rider who briefly gives with both reins while the horse is in a good canter and the horse maintains its rhythm, balance, and posture without falling on the forehand is riding a horse in self-carriage. A horse that immediately falls on its forehand when the reins are softened is not yet in self-carriage and is relying on the contact for balance.
Developing self-carriage through rein work requires the rider to progressively reduce the support the contact provides — to give more frequently, to maintain lighter contact between aids, and to ask the horse to organize itself for progressively longer periods without rein support. This can only happen alongside adequate leg and seat work that is developing the hindquarter engagement and core strength that self-carriage requires. Reducing the rein support without developing the physical capacity for self-carriage simply produces a horse that falls on its forehand.
For both dressage and Western riders, self-carriage is the benchmark of advanced training — the dressage horse that performs piaffe in self-carriage and the reining horse that circles in self-carriage on a loose rein are both demonstrating the same fundamental achievement: a horse that has developed the physical and mental organization to maintain its own balance and posture without depending on the rider's hand for support.