Film/video cameras have both a shutter speed (in celluloid cameras this is referred to as the "shutter angle") as well as a frame rate, and these two functions can be adjusted independent of each other. In this case, the video was shot at 30 frames per second, but each frame has only been exposed for maybe 1/1000 of a second (imagine a still camera with a very fast burst mode). Because of this quick shutter speed there's no motion blurring (not even in the shadows, which move at the same rate as the blades themselves), and with a little fine-tuning of the shutter phase it's a relatively simple task to make the blades appear stationary. (You sometimes see this in car commercials where it looks like the wheels are turning backwards even though the car is moving forwards)