Notice how the pitch changes as the cyclic is, very probably, being pushed forward. The blade increases its pitch adding power as it passes on the aft of the rotor disc, which pushes the helicopter forth.

I would love to see how they attached the camera. I kept waiting for it to be shot away from the helicopter. Credits go to Helicopter Enthusiasts for the video and Gareth O'Driscoll for pointing it out.

MI-8 main rotor blade in flight video