Coincidentally I came across one of these in amongst my box of bits. It's never been used and I did wonder how it works. I think mine had a diagram or instructions attached to it, I'll have another look later.
I have roughly worked out how it all works. The top bit when pushed down allows the brake cable to move and when it isn't it's sorted of clamped in place. I'd be interested in what the instructions say if you can take some more photos.
