Ah, I was misunderstanding your question. I think I had the same question, which, if I understand properly this time, is not necessarily different than asking how the switch turns the light off. I believe the video only showed how it turns the light on.
I guess the system is programmed to recognize that the light is on, and detect whether there are any successful receptions of the pulse within a certain period of time, and if not (which would occur if the plastic was blocking the pulse), to turn the light off.
This is just a guess.