If we want to look at the body mechanics of the back up we have to look at the bend of the body.
It would be great to just say the magic words and have a horse drop into reverse but it does not always work that way.
I have a couple of horses here that were just about born running backwards and a couple the I would like to hitch up to a backhoe.
Let's assume that your horse is one of the tough one's.
If you were to bend a horse to the left from the ground and get him to step with his left rear foot,then the foot would step back as the pivot began.
If you bent the horse to the right and the same thing happened,then you are on your way.
So I feel that the backup begins from the ground and from the pivot.
A horse can get kind of "bound up" and need to bend and pivot and then expand it into the back up.
I just posted these pictures on another thread but I think the maneuver is part of the foundation of basic training.
In these pictures you can see the horse bending to the right and the first REAR
step that will happen is with the right rear foot as the front feet step across with the pivot.
This is the very beginning of the back up.
In the beginning the handler hopes for one step.
Bend to the left....step.
Bend to the right ....step.
You have to watch this and not over bend the horse to the point that they step forward with the outside foot instead of stepping back with the inside foot.
If there was a next picture it would show this horse picking up the right rear foot to step back but I missed the shot.
I will try to get that moment next time.