Of course, the real explanation here is that this is a stroboscopic effect, aka the wagon wheel effect — the rotor speed and the film speed are matched (to some ratio) so there is no apparent movement of the blades for each frame of the camera.
Try it yourself and alias your monitor’s refresh frequency
That’s pretty cool, how fast are the rotors on something like that?
315 rpm.
Turns out I had misidentified the helicopter. The one in the video is an Mi-2, and it turns out to have a rotor RPM of about 240. A 30 or 60 Hz video recorder divides 240 evenly.