The frameline selector switch is used for preview only. After you let go, the framelines for the lens that is mounted come back.
I always thought the frameline selector was the sneaky ploy for getting people to buy lenses. OOOhhh look at those framelines! I need a 9cm and 13.5cm lens! Without the pre-view, you would not have known what you are missing.
I think it should be possible to latch the frameline preview lever into any of the three positions. Would make it unnecessary, for instance, to file the mount of 40mm lens to bring up the 35mm frames... Did Leica miss a bet? And Zeiss too?