you mean idle curiosity? :P
i believe it's a system that uses a light sensor on the bottom of the rear view mirror to detect how much light is hitting the mirror. there should be another sensor somewhere (the front?) that determines ambient light conditions, so the internal computer can determine the difference between the two sensor outputs. the difference in the two readings is, then, the light coming from the car behind you.
not sure the exact relation, but the difference in sensor readings adjusts the voltage that passes through a gel-like mesh in the mirror itself, which is an electrochromic material. that pretty much dims or lightens the mirror as needed.
sorry, it's not a detailed explanation, but hopefully that gives you the jist of things.
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