A basic rule of Newtonian physics is that for every action there is an equal and opposite reaction. So when you have the action of the rotation of the main rotor blades of a helicopter – the reaction will cause the body of the helicopter to move in the opposite direction. This torque reaction is offset by anti-torque systems like a tail rotor, Fenestron or NOTAR system. If the helicopter has more than one main rotor then they will counter-rotate to offset each other’s torque and remove the need for a tail rotor etc.
The video below explains the helicopter torque reaction and the need for an anti-torque system:
- FAA Helicopter Flying Handbook Chapter 2 – Aerodynamics of Flight
- FAA Pilot’s Handbook of Aeronautical Knowledge Chapter 4 – Aerodynamics of Flight