They work just like the Taron restraints (first example that came to mind):On another note I've been staring at the photo for a while and I can't figure out how the harness works or what part hinges. It looks almost too compact to fit a person in but I imagine that's mainly the picture angle playing tricks.
Rather than one curved piece of metal on either side of the rider, however, they have some sort of connection piece (I think with a seatbelt or something - hard to tell?) in the middle. Once your eyes "figure it out" it'll become a bit more obvious - took me a while to suss it out earlier too as I thought there was another hinge in the middle.