You got it ThAt GuY120... There is a mounting plate that attaches to the rear bar that is used to mount the canopy. The tail boom then mounts into the mounting plate. As you noted, the mounting plate (and attached boom) are "hinged" so that the rear of the boom can lift up. The O-Ring is used to keep the boom from lifting up. The nice part about this design, is that it provides "give" if you land hard on the boom and tail... it just lifts up rather than break. I have the BoomTown boom and have been very pleased with it!