To ask the horse to back up is to ask him to go forward. Sounds like a paradox does it not? Remembering that the bridle is a tool that blocks unwanted direction, ask the horse to move forward into hands that do not pull back, but block from going forward, left or right. Have your legs ready to block any sideways movement. The only “open door” available is to go backward. Be patient when first teaching your horse to back up. Only ask for a step or two and then give lots of praise.
The bridle is a tool which sets boundaries telling a horse where not to go. It is used for blocking unwanted direction. The right rein should not be pulled to steer the horse to the right. This will throw the horse off balance. Instead it blocks the horse from going to the left. The left rein does not pull to steer the horse to the left. It simply blocks the horse from going right. Pulling creates stress in the horse and does not elicit a sense of trust. Is your horse in good hands?