Backing a horse or pony in harness is an advanced move in driving. I would not suggest trying to back until you and your equine are comfortable in all other aspects of driving, especially the halt.
You never want a horse in harness to use backing as an evasive move, which is one reason you always have your whip in your hand, to give him some forward encouragement if he ever back without being asked.
Here is a good article on backing up. All of the articles on this website are very good, I am going through and practicing all of them that I can over the winter. http://coachmansdelight.com/CGuidePa...=GUI25090&k=58