Wait, if you press the key for ‘k’ and the three way switch moves from lightbulb contact to battery contact. As the video explains the letter is rewired from one letter to another letter, say ‘x’, and when the circuit returns to the three-way switchs it’s now directed to the letter ‘x’ which is not pressed and in the lightbulb contact position.

So what happens if ‘k’ randomly returns to itself and the switch is in the battery contact position and not the lightbulb position—no light?