According to H.T. Part 6

Once again, don't forget to go through your ground exercises before you mount your horse. Once mounted our horse should be getting accustomed to us being in the saddle. He is getting more responsive because his mind is not just thinking about us on his back. Today when we mount we are going to ask him to back. Before we ask him to back we should make sure he has his feet up under him. It is very difficult for a horse to back if his legs are in a stretched position or if his legs are not under him straight. There are several different ways to ask a horse to back and I have tried several of them. I will not go into all of them but will try to give you one that works well for me. First remember how you ask him to back from the ground. Start with saying the word..back...take the slack out of your reins...keep the reins low on his neck...add a little pressure in the reins. Do not pull back quickly or severely as it will cause him to throw his head back as he will try to get away from the pressure in his mouth and will forget everything else except getting away from you. We want to keep all of his experiences happy, so that he does not associate backing (for instance) with pain. It is not important at first to get him to take more than one step. As soon as he takes the first step release the pressure (the same as we would from the ground) He may step to the side, but that will be O.K. for now, we just want him to move his feet back when we ask. After a few times of asking for one foot to move...ask for two steps...this will be over a period of time. I don't like to dwell on one thing for long periods of time. I don't want the horse to become aggravated with the same thing over and over. So, if I get a little bit of what I want from the horse, I will move on to something else, and come back to the backing later. Since I don't want to put the horse up for the day, this lets him know that I was happy with what he gave me because we moved on to something else. If you have played with your dog, after a few minutes he will become bored with running after a Frisbee, but if you move on to a nice walk on the trails, and come back to the Frisbee later, he will go right back at it again. This is what we want to do with our horse also. Notice when he becomes aggravated or bored with something you are teaching, be prepared to move on to something else he knows and come back to it later.