The button when pushed has this antireflective stuff in it. So when your driving at night people with bright lights that blind you will be turned into a greenish tint and even the brightest light wont blind you. This way you dont have too flip the mirror up and down when some Jerk has his brights on you and is tailgating you.
You wont notice it working during the day, but at night you should unless its broken. Turn it on give it a few minutes and drive around at night, when someone is behind you it should look as if the lights are greenish instead of white.
2013 6.2 V8 Camaro SS 3,000 miles :-)
1998 5.9 V8 Limited Platinum ZJ 151,xxx miles
1998 4.0 I6 Laredo ZJ 207,xxx miles Yay !