What is the correct way to hold the reins when mounting and why does rein position matter?

Rein position at mounting is a specific safety and training detail that Clinton Anderson addresses because incorrect rein position at the moment of mounting is one of the most common causes of horses spinning, bolting, or backing when the rider mounts. Anderson teaches holding both reins in the left hand with light, even contact — not so loose that the horse can walk away before the rider is seated, and not so tight that upward pressure on the horse's mouth when the rider rises causes the horse to back away from the pressure. The common error of holding the reins very short and tight on the assumption that this provides control actually causes backing, because the rider's rising weight pulls upward on the mouth through tight reins, and the horse backs to relieve the pressure. The left rein should be slightly shorter than the right, so that if the horse moves forward the rider can pick up the left rein and immediately tip the nose left into a one-rein stop without needing to reorganize the hand. This is the one-rein safety position Anderson builds into the mounting sequence as a standard rather than an emergency option. The right hand goes on the horn or pommel — not the cantle — and pushes straight down rather than pulling back. This keeps the saddle level during the mount rather than torquing it left. Once the rider is seated, both hands take up even contact before any movement is asked. For horses that back away from rein pressure during mounting, Anderson's correction is to immediately release rein pressure and re-establish soft flexion from the ground before attempting to mount again — confirming the horse is light and not braced before the riding position is taken.