When the trigger on the Zapper is pressed, the game causes the entire screen to become black for one frame. Then, on the next frame, all valid targets that are on screen are drawn all white as the rest of the screen remains black. The Zapper detects this change in light level and determines if any of the targets are in its hit zone. If a target is hit, the game determines which one was hit based on the time of the flash, as each target flashes for one video frame, one after another.