The flirt pole is a great conditioner as well, especially for the dogs that go nuts over the object on the other end. Like all conditioning activities and conditioning tools one must look at what the dog will be doing in competition. The people version would be 'sport specific' training. This concept can be applied to the dogs as well. The dog sees what he wants and explodes after it with all he has targeting his bite to a moving object. Sounds a lot like like scratching to a hold or swapping one hold for another. The flirt pole can lead to what is called a fast mouth dog. The changing of direction, all out one way and in a blink all out the other. This mimics the swapping of a hold or defensive posturing. These movements are bursts, fast twitch muscle fiber, going from all out to nothing (or at least it appears nothing as they focus to move but every fiber of their being is focused on biting what is on the end of the string) which mimics a flurry. And the last it teaches the coordination to bite the intended target. The hardest, fastest mouth is worth nothing if it dives in and comes up with carpet in his mouth. And for me, and the most important, his feet is on the ground and he is not only supporting his body weight but he is changing his direction and propelling that weight forward or whatever direction the target travels.
The Jenny follows suit with the idea of the best conditioners are the ones that keep the dogs feet on the ground. His scratch or his push or his driving is done carrying/propelling his own body weight plus some of the weight of the opponent. On show night the floor will not spin out from under him and propel him into a hold or a deep defensive position. He must do that on his own and it is better for him to be prepared to do so in the same format. Basically when on the ground the dog is always under load in every thing he does, where as the slat mills, especially the really nice ones, he is just keeping up with the belt. There is nothing wrong with a slat mill but if it is so free that there is slack in the chain hook-up there is a lot of wasted motion. If a slat mill is used there should be tension on the chain as the dog is propelling at least part of his weight forward. I personally prefer the carpet mill over the slat mill but that is an entirely different subject.
The first Jenny I ever saw was in the mountains of NC. It was a turn table made out of square steel tubing. The table must have been a 10 or 12 footer. There were two arms that could be added to opposite sides that converted the table to a Jenny in just a few minutes. It was nice. He had a male that could make the arms look like helicopter rotors. That dog could move. The next best one was hidden right in the middle of a horse farm. This place had a motorized horse walker. Kids were learning to ride horses. Horses were put on it for daily exercise. At night the chain was taken off the motor and it was used by the dogs. That is the biggest drawback to the Jenny is its size. The bigger the better but it takes up a ton of room and with that much room, for most, that is very little privacy.
The first Jenny I saw used counterweights on the opposite arm but also had differing lengths of chain to drag for resistance. I built one similar to that one with a table some years back. I liked it a lot and thought the dogs worked it rather well. The key is to start them off young getting the accustom to chasing something they want. I liked the turntable on it as well. Back then we lived on the end of a dirt road with no one around at all. The land leading to us was developed and somethings can't be explained away to the neighbors. So I traded in the jenny and stuck to the carpet mills, hand walking and explosion work. Those can be a lot more private. EWO