What does flipping that little plastic tab under your car's rearview mirror actually do to stop blinding headlights?

The tab changes which surface reflects the light

In a conventional rearview mirror, the reflective coating sits behind the glass. In the normal daytime position, light from a car behind you passes through the glass, reflects off that bright coating, and returns to your eyes as a clear image.

Flip the small tab, and the mirror pivots to a slightly different angle. That shift redirects the strong reflection from the coating away from your eyes. Instead, you see a much weaker reflection from the glass’s front surface—the faint image that is usually overwhelmed by the coating’s reflection.

Less glare, not a blocked view

The headlights are still visible, but their image is dimmer, reducing the harsh glare while preserving enough of the scene to see traffic behind you. The night setting can make the whole view less bright, so flipping the tab back restores the clearer daytime image.

The trick works because a glass surface reflects a little light even when most light passes through it. The tab simply changes the mirror’s angle so that faint reflection becomes useful.