In short, the TV emits the light pulse and the gun detects it, not the other way around. But that's for the gearheads. At the Straight Dope we're more interested in the deeper meaning of it all. You pluck a card at random from a deck, then return it. The magician then displays four cards so that only you can see the faces.
None is the ace.
She places the four cards facedown in a square. By sleight of hand, the details of which need not detain us, she replaces one of them with the ace. If you pick the ace, she discards the other card. If you don't pick the ace, she discards the card you picked. Of course it is.