Today I finally understood how the coupled rangefinder mechanism works on Leica-style cameras and how the camera body "knows" the focus distance of the lens.
The lens' focusing mechanism moves a piece of the lens that extends from the back of the lens into the camera body and presses on a little round part called a cam and depending in how much the lens presses on it, the rangefinder optics can be adjusted to show the two images diverging or converging.
