How exactly does the Zapper work?

[GameMaster]

The NES Zapper is a pistol-shaped electronic device sold as part of an original NES system. Early Zappers were gray, but later the color was changed to orange. It allowed players to aim at the TV and, depending on the game, shoot pixelated ducks, clay pigeons, gangsters, terrorists, and many other obvious targets.

When the trigger was pulled, the game blanked out the screen with a black background for one frame, then, for one additional frame, drew a solid white rectangle around the sprite you were supposed to be shooting at. The photodiode at the back of the Zapper would detect these changes in intensity and send a signal to the NES to indicate whether it was over a lit pixel or not. A drop followed by a spike in intensity signaled a hit. Multiple sprites were supported by flashing a solid white rectangle around each potential sprite, one per frame.

Pretty slick.