Thinking of picking one up to help overload my chest once a month. If you have no clue what I am talking about just google it or watch the video on YouTube.
I presume these serve the same or similar purpose of bands and chains.
It appears to give you more assistance at the bottom of the movement, with decreasingly less assist towards the top - the idea being that the device provides adjusting resistance, thus offering maximum workload on the stronger and weaker parts of your ROM.