A mirror consists of two different components. A glass which reflect about half the light itself and offers protection and structure to the mirror. And then a back cover of a reflective metal which reflects the rest of the light. Normally the reflective back surface is painted on and never comes off. But in a car rear view mirror this is a separate piece which is spring loaded towards the glass. By flipping the leaver you are moving the backing away from the glass. So only the glass is reflecting light making the reflected image darker.
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