The build occupies the first part of the twenty-four minutes: a laser aimed at a small mirror on a vibrating membrane, reflecting onto a wall, so a single point of light is drawn out into a Lissajous-like figure by the motion. This is the cheapest way to get large, projectable cymatic imagery, and the rig is shown properly rather than gestured at.
The second part sets out what he calls the Stillness Theorem: that two frequencies played together give a static figure if and only if their difference divides both, giving 60 Hz and 90 Hz with a difference of 30 as the example. That is his own coinage, not established terminology, but the underlying condition is the familiar one for closed Lissajous curves — the two frequencies must be in a whole-number ratio. Framed that way it holds; framed as a new theorem it overstates.
The rig is the reason to watch.