I don't have the time at the moment to put a sketch together but I've used two different systems.

System 1) (simple - crew only). I have a bunji that is doubled up inside the beam (little blocks tied to the castings on either side that the bunji goes through). On the bitter ends of the bunji there is another small micro block. I have a 3/16" line that goes through the blocks on the ends of the bunji and terminates on the rear beam. On the end of that line is a stainless steel hook and a v-jam cleat. The bunji pulls this line, doubled over, into the rear beam and pulls the v-jam cleat/hook up tight to the hole in the beam end cap. When the crew is on the wire he can bend down, grab the hook, extend it and hook it somewhere onto his harness. This won't keep him at the very back of the boat but it will keep him from going much forward of the beam if we stuff.

System 2) crew and skipper (but more difficult to use at deep wind angles). The other system terminated similarly in the rear beam but the line carried forward and tied solid to the front beam. Both crew and skipper have cam cleats on their harnesses (I had plates welded to the spreader bars for the cleats).When on the wire, you both step under the chicken line (which is pulled tight from the rear beam to the front beam) and drop it into one of the cam cleats on your harness. As skipper, I usually depended on my crew to hook me into this system. The nice thing about it is that if push really came to shove, the cam cleats would let go if you put them at a really bad angle so you would be less likely to be dangling from the top of a capsized boat.