I have one of those Union BB generators, sitting in a box and awaiting being used. The control lever works via a cable, like a brake or shifter cable, that attaches to the generator wheel and snakes up the seat tube, to the lever. The generator wheel is spring-loaded, with the relaxed position pushing it forward against the tire. To turn the generator off, you pull up on the lever and turn it into a locking notch, which pulls the wheel off the tire. To turn the generator on, you release the lever from the notch, let it drop down into its plastic sleeve, which allows the spring to push the generator wheel into the tire. It basically works like a derailleur/shifter interface.

Although my generator has "Union" on the parts, the instructions on mine are from another company called Basta. I had read somewhere that Union acquired both Basta and Soubitez somewhere along the way.

I used to have one of those Sanyo BB generators back in the early 80s, which crapped-out after a few years.