I had a gelding that had a stifle injury and I would ground drive him because longing would put to much stress on the injured leg. If you need controlled exercise then you can work in straight lines and ground drive. When we warm up our horses we will ground drive them sometimes instead of lunging them. You can do everything you would do when you are riding. You want to be behind them but off to the side a bit so they can see you. Being behind the girth line, you are driving them forward so they should work as if you were riding them. Some horses will want to turn towards you and if they do that just do your best to stay behind them and work your reins to get them going forward. You can use this to train them on different bits, especially if you don't know how the horse will react. You can teach them to turn towards the rail and turn on their haunches. There are a lot of uses and I use it pretty often.