It is a brake, but like none that I've seen. I like the way it works.
There is another very similar product that has been on the market 10+ years. My tired old mind can't remember the name, and I can't seem to locate a link either. It operates in exactly the same way, as I recall, only the apparatus hung from the boom is more spherical in shape (while synthetic in material, it actually looks a bit like a lignum vitae block from an old sailing ship), with holes through which the control line runs circuitously to create friction.
Maybe somebody with a better memory will recall the name...