Connects to the bottom of the jet pack.
I haven't read about or seen it before
It is not really necessary. I was going to add this to my setup & I found it was going to be hard to reach. It keeps the jetpack from swinging from side to side but unless you are doing stunts it is not really a concern. A piece of Velcro there would serve the same purpose. I actually put a bit of weatherstripping on the back of my pack so it doesn't rub against the back plate & it provides enough friction that the pack doesn't sway.
I had been calculating what to use between the pack and the backplate. Personally I'd not want those parts riding on each other either.....
Well I can vouch for the weather stripping but I think Velcro would work just as well. Just put a big looped piece one one side & a smaller hook piece on the other. The hook part is slightly abrasive.