The chain I used on my jenny was not a normal chain, it was a ship anchor chain that I could wrap up or drop. It acted as a counter weight opposite the dog in either scenario, but by dropping it the dog would drag a few feet of it adding a resistance element to the catmill work. This is no different than when I used both a slat and carpet mill to acheive two different kinds of training, but once again much more natural than both of those tools.