Alpha waves refer to the measurable 8-12Hz waves in the electromagnetic field coming out of the human head. They are the clearest "signal" we can read out with eeg ("electro" "encephalo" (brain) "graphy") and usually peak in power over the back of your head.

They are also by far the biggest (measurable) eeg signal change you can manipulate intentionally (other than motion artifacts). Closing your eyes or focusing your attention inwards reduce the power of those oscillations so much it's visible right away just looking with your eyes at the signal trace.

It would be very straightforward to program in decision points to the light show where someone can select an option by tuning their attention inwards or outwards.

TLDR: It's the "press X to doubt" of human-computer interfaces.