That's a cool illusion.
That video must have been shot with a video camera that has a frame rate with the same speed as the rotation rate of the main rotor. It would give the illusion that the main rotor is standing still.
You can see that the tail rotor appears to be rotating slowly which means it is at a slightly different rate from which the camera is filming.
You can definitely hear the sound of the rotating blades in the audio, so you know they're turning and not standing still.
You can also get this illusion with airplane propellers.