Leica Optional Viewfinder for WA Lenses

I never had understood this. Ran across a Leica viewfinder for the 21/24/28mm lens that appears to mount on the "hot" shoe of the M2, M4, etc. How do you know what the coverage is buy looking through it since normally you look through the viewfinder of the camera itself? I wouldn't think the focus window would be visable by looking through this type of viewfinder.

Look through the camera viewfinder to focus (not that you need to much for a 21mm lens). Then move to the external viewfinder to frame.
Or vice versa. And repeat if you want.

Well, that makes sense now...thanks. I've seen photos of Eisenstaedt with those devices on his cameras and often wondered how they worked. I learned something today.