The way the gun worked was, when you pulled the trigger, the screen flickered momentarily. All other details disappeared and were black, and the hitboxes of the ducks were represented in white. The gun had a light sensor in it to detect whether what it was pointed at was light or dark; if the result was 'light', then the game figured you were pointing it at something white (i.e. the hitbox) and registered a hit. This meant you could trick earlier versions of the game into always registering hits by pointing the gun at a lamp when you pulled the trigger, so it always registered 'light' (albeit at the cost of eventually burning out the sensor in the gun to the point where it wouldn't recognise the meagre light put out by an '80s tv screen).

Nintendo fixed the problem in later designs of the Light Zapper by incorporating a slight delay in the display of hitboxes and having the gun detect the dark-to-light shift rather than just 'light' in general; they assumed people wouldn't be hardcore enough to turn their lamps on at the exact same time as they pulled the trigger. No point in cheating if it's harder to pull the cheat off than it is just to play the game normally, after all.

D.F.