To back a horse, you don't want to pull the reins backward...that causes the horse to open his mouth and throw his head upward, hollowing his back.
What you want to do is keep your body in a perfect upright position and remain there as you shorten the reins with your hands...don't pull back...just shorten the reins until you feel the horse shift his weight to his hindquarters.
Once the horse shifts his weight, you must hold your hands steady...don't lengthen or shorten the reins.
While holding the horse in that frame, begin to squeeze with both legs...the horse will back because his weight has shifted rearward. The horse will continue to back as long as you apply leg pressure. When your leg pressure is released, the horse will stop backing. If you do not change the length of the rein, and reapply leg pressure, the horse will begin to back again.
If you lengthen the reins, the horse will move forward on leg pressure.
If you train your horse to back on leg pressure and not by pulling back on the reins, the horse will not open his mouth and he'll lower his head rather than lift it.
To make it even easier for your horse, lift your weight off your horse's back by standing just a bit in your stirrups...this will make it easier for your horse to round his back upward.
Learn how to teach your horse to back properly - take the online course Training Performance Horses - Click here for more information.
HOW TO BACK A HORSE