This disc tilt will only happen with an airflow over the disc (forward speed or wind). If you, say, raise the lever the pitch angle will increase equally around the disc. However, because the advancing blade has a higher airflow then the retreating one it will experience a greater increase in lift than the retreating blade (remember 1/2 rho V squared S). As always happens, the effect is noticed 90 degrees on, so the disc flaps back. Lowering the lever has the opposite effect and the disc flaps down.