How does Anderson teach the one-rein stop as it applies to the moment of mounting?

Clinton Anderson's one-rein stop is one of the foundational safety tools in his program, and its application to mounting specifically is something he teaches from the very first ride on any horse. The principle is that a rider who has one rein picked up at the moment of mounting has immediate control available if the horse moves — rather than needing to find the reins after the fact when the horse is already in motion. His mounting sequence specifically incorporates the one-rein position: the left hand holds both reins with the left rein slightly shorter so that if the horse moves forward, picking up the left hand immediately tips the horse's nose left and disengages the hindquarters. The right hand is on the horn or pommel. The rider rises into the stirrup already positioned to respond if needed. Anderson teaches that the one-rein stop at mounting is not about expecting something to go wrong — it is about being in the correct position if something does go wrong, which removes the danger from the equation. A rider who mounts with loose reins and no immediate control available has given the horse a window of time between the horse moving and the rider finding the reins in which anything can happen. He also uses the pre-mount one-rein flexion check to confirm the horse is responsive to the rein before weight goes in the stirrup. A horse that flexes softly has demonstrated that the one-rein stop is available. A horse that is stiff or resistant to the flexion is telling the rider that the one-rein stop may not work as planned — which is information worth having before getting on rather than after.