As Dutch says, with some cars, a very careful driver, and smooth flat terrain, it may be possible to back short distances, anywhere from 3-10 feet. The general prohibition against backing recognizes that doing so is risky for the average person and vehicle. The front wheels on the car can and will turn quickly and sharply, putting major stress on the tow bar, base plates and the car front end. If you aren't watching closely (which isn't always easy to do), you can bend something before you even realize it.