My guess is that washer takes up and lets out string as he wraps it round.
Yes, its used to shorten the string so that you can go from long string, no slack, half brace, full brace, depending on how much you wrap the string, and it works well.
Thanks for the replies and your advice so far, guys. I will make a tillering gizmo/cheater tool before I go any further. One question:
Should I fix the hinge in the left limb before I start tapering the limbs more or can I do it afterwards? I plan on tapering the limbs from mid-limb width now because I feel it will be easier.