When I got my R3a it was set by default to 40/90 but the switch will go to 50 or 75.
I haven't touched the switcher yet but wondering how it works, what happens if I switch it to something else with the 40mm still connected, etc ... What is actually going in inside the camera when the switch is made.
Yep. It only effects the framelines that are visible in the viewfinder. It can be a great tool for "previewing" different focal length compositions before changing lenses.
The frameline selector is in NO WAY attached to the lens or shutter, so you can flip it at will, regardless of what lens you have on the camera. All it does is change the brightframe visible in the viewfinder window.