This was how I was taught:
Never had a problem with a horse wanting to back up irrationally. Mine aren't too fond of doing it unless there is a reason, and then they usually are waiting for me to figure it out. All they need then is "permission". In essence, you don't ask them to back up so much as ask them to move, only not forward or sideways. Have no idea if it is in any way related to a rein back.
