It looks very nice, but i have 2 doubts:
1- Does it help to prevent lower back pain?
2- How does it affect the performance of jumps with rotations? It seems to be strange...
For full disclosure, I've never used a dynabar, I use a rope tied across my harness bar that a attach to my chicken loop with a carabiner. It is like a poor-man's dynabar. It isn't nearly as elegant a solution as the dynabar, but it works and I think I can answer your questions. I'm not trying to hijack a dynabar thread to provide a DIY alternative. Dynabar looks like a really good product and once I have any money to invest into more kiting equipment, I will probably buy one.
1.) Back pain (for me) is caused by the harness, not the spreader bar. If you are having lower back pain, get a seat harness. Seat harnesses suck because of the poor mobility and because they won't twist around your body at all, making toe-side riding (or any contorted riding) difficult and uncomfortable. Dynabar (or my cheap-ass rope setup) will allow you to ride with much greater mobility and comfort in a seat harness, and the seat harness (which would suck without the dynabar but is comfortable and mobile with it) will help to prevent back pain. Basically, it allows you to use a seat harness without having to accept the suckiness that normally comes with seat harnesses.
2.) I haven't noticed any effect on my ability to rotate during jumps; left to its own devices, it seems to settle at the center, right where it would be normally, but it is easy to get over to the sides if you want it to. If you are really into rotations and lots of them, I don't know, maybe it wouldn't be so great, but it hasn't affected my riding negatively at all. Toe-side riding is way more comfortable (and jumping from the toe-side edge is vastly easier) and heel-side riding can be pretty nice too-normally the harness will settle into the center, just where it would be with a normal spreader bar, but if you want to really edge, it will swing over to your back foot side, allowing even greater upwind edging.
If you only ever ride heel-side and you aren't at all into carving/waves, the dynabar (or rope set-up) probably won't do much for you. If you ever ride toe-side, it can really change the way you ride.
I really like my poor-man's dynabar, and once I have any money to put into kiting gear, I'll probably get a real one. My rope works pretty well, and it is fine for the short term; if you don't know whether or not you'd like a dynabar or can't afford one, just tie a rope to each side of your spreader bar and use a carabiner to attach the chicken loop, but before you do this consider these two potential problems (inherent to my rope, but not to the official dynabar): This cheap-ass setup screws up the throw-length of the bar and puts it a bit out of reach (dynabar does not appear to have this problem, which is why I'll probably get a real dynabar instead of continuing with my rope setup). Also- check your quick release. My quick release occurs above the chicken loop, so if it carabinered in, it works just as well as if it is on a harness hook. If your quick release is different, and could get caught on a carabiner, my rope set-up could kill you. Again, the official version (which I've never tried, but have looked at a lot of pictures) would not have this problem because it just uses a regular harness hook.