Refer to figures.
To turn the main rotor the engine (power on) must exert a turning force or “torque” on the main rotor shaft.

According to the Newton’s 3rd Law: Every action must have an equal and opposite reaction. So, the effect of applying torque to the main rotor shaft, is an equal and opposite rotation of the fuselage.

Conventional helicopters use a tail rotor and some others, the NOTARs, a variable pitch fan at the front of the tail boom, to counterbalance the torque reaction through the anti-torque or yaw pedals, producing a horizontal tail rotor thrust.

Since the horizontal thrust which counteracts the torque effect is produced from one point (the tail rotor) and towards one direction, then the helicopter will have the tendency to drift.

Because of the tail rotor drift (or translational tendency), the helicopter will always drift towards the side of the advancing blade (for a clockwise main rotor the advancing side is left of the pilot).

This can be corrected by tilting the rotor disc and thus the total rotor thrust away from the direction of the drift and producing a horizontal component of total rotor thrust to the opposite direction, the retreating side (for a clockwise main rotor the retreating side is right of the pilot).

And can be achieved by:

• Manual stick input.
• Rigging the controls so that, when the cyclic stick is in neutral position, the rotor disc is actually tilted by the correct amount.
• Mounting the gearbox so that the rotor mast tilts slightly.