The bright reflection is the reflection off the silver backing of the glass in the mirror (i.e. the normal mirror reflection). The dim reflection is the reflect

The bright reflection is the reflection off the silver backing of the glass in the mirror (i.e. the normal mirror reflection). The dim reflection is the reflection off the surface of the glass (the same as the reflection you see of yourself when you look at a window when it is dark outside and bright inside). Flipping the "switch" on the mirror simple changes the angle of the mirror by the exact amount needed to reflect the light from your rear window into your eye off either the silver surface or the glass surface.