Here is a great video of Lee Priest training rear delts in his home gym on the Powertec Leverage Pec Fly Rear Delt. This is perfect demonstration on how to perform certain isolation movements.
You will notice that Lee does not go all out with ridiculous weight. Instead he is just using a weight that he can control with ease and just knocking out the repetitions. You can even hear comment about the "burn" at the end of the set.
Now this machine is currently no longer in production. It has always been one of my favourite machines and has been stopped before and returned. I have managed to stock pile some and have got some more coming in, so we should have a few of these pieces in Australia for some time. With a bit of luck this machine will make a come back very soon.