A friend of mine had one, and after we saw how it worked, we quickly cloned it. It was ridiculously simple, a string, and a joystick, the rest of it was just set dressing. It was a very simple matter to put two screw eyes or hooks on a plastic model, that and some 12# monofilament and you were set. We had some of them that went over 100 feet with about 20 feet of drop, the far end was in a tree and the near end was over a hill and halfway across the front yard.
It was trickier than it first appears, particularly with very long runs - you could get it really cranking and had to slacken up to drop the slope at the right time so it slowed down by the time it got to the end, and not stop halfway down.
Brett