Those are great. Mike has one on his houseboat. You connect a 15 to 25 lb weight to a rope and handle combination on a pully at about the same angle as the tower / rope combination while you are riding. Then you use it as a training aid. It simulates real riding and helps with spins and handle passes.
You can also make one at Home Depot for like $18. A great training aid as it really allows you to train your balance as you spin. My friend (RTA where you at?still sick?) has one & he uses it all the time. Lots a fun too.