Not often I get to explain something to Crab. Plenty of the other way around. All assuming of course that this isn't a massive waaaaaa.
Someone much cleverer will give you a better explanation, but this is how my understanding of the frame rate works...
The video is made of a series of stills (30/s is typical). If the 'shutter' time of each of those stills is quick enough, the blades will not be blurred in each of those frames.
Take the example of a 3-bladed rotor doing 600rpm. That means 10 Rev per second or 0.1 seconds per revolution. With 3 blades, that means the time between blades going through any particular point on the disk's path is a third of that, or 0.033 seconds. If the video was at 30 frames per second (call it one frame every 0.033 seconds), the disc would appear stationary. This is because each of the stills of the disc would show the blades in the same place.
Now increase the Nr by a little and the video would show the blades turning slightly in the correct sense, because each blade is able to get a little bit past its predecessor in 1/30 of a second. Slightly below 600rpm and the blade will appear to be going backwards because each blade has not quite got to the position of its predecessor in 1/30 of a second.
This is also true for multiples of 600rpm. At twice the rpm, the blades appear stationary, but this time because in each frame, the position of a blade matches that of the one 2 ahead.
Simples. Also called Strobe Effect.