What is Clinton Anderson's pre-mount safety check and why does he do it before every ride?

Clinton Anderson performs a specific pre-mount safety check before every single ride, and he teaches it as a non-negotiable habit regardless of how well-trained the horse is or how familiar the situation. His reasoning is that horses can change significantly from one day to the next — a sore back, a digestive issue, tension from a change in environment, or simply a fresh horse after time off can mean the horse that was quiet yesterday is not quiet today. The pre-mount check begins with flexing the horse's head to each side from the ground using the rein. Anderson teaches picking up the left rein and asking the horse to tip its nose softly to the left stirrup, then doing the same on the right. A horse that flexes softly to both sides is soft in the neck, responsive to rein pressure, and giving the handler a preview of how it will respond to rein aids under saddle. A horse that is stiff, resistant, or braced at this simple ask is communicating something worth paying attention to before getting on. He then checks the cinch is tight enough — he specifies being able to fit two fingers under it but not more — and rocks the saddle gently to confirm it is secure. Only after both the flexion check and the equipment check does he put his foot in the stirrup. Anderson teaches that the flexion check specifically also establishes one-rein control from the first moment of mounting. If the horse moves when he puts weight in the stirrup, he already has one rein picked up and can immediately disengage the hindquarters and stop the movement. He is never caught flat-footed at the moment of mounting because the check has already put him in the correct position to respond.