The bluetooth symbol is a combination of the early runic characters 'H' and 'B', Harald's initials. His monogram, if you like.

Damn you, Juliano Schroeder, I wanted to say that but you beat me by a day!

It is an example of the stroboscopic effect.

The frame rate of the camera in the video is a sub-multiple of the revolutions per minute of the rotors. We may not be seeing the same blade in the same position each time - we are seeing a different blade caught in the same place. Since the blades are visually the same, we do not notice any movement.

The more blades there are to the rotor, the more likely it is that the speed will synchronise in this way.