In a running airborne helicopter, the turning main rotor experiences drag that would slow it down if the helicopter did not supply torque to keep it running. In supplying this torque, the helicopter experiences an equal and opposite reaction (Newton's third law), which would tend to rotate the helicopter about the mast in the opposite direction to the main rotor, unless the aircraft uses some form of anti-torque control.
During an autorotation, the helicopter is no longer supplying power to the main rotor and therefore experiences no torque reaction; in fact, it may experience a rotating force in the same direction as the main rotor, due to drag in the one-way bearing.
During the initial spool up of the helicopter, unless soft start is used, or the head speed is not increased gradually by slowly increasing the throttle, the helicopter may spin around violently, as the tail rotor will have insufficient speed for good control authority.
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