Try this:
The main rotor is being pushed around by the engines which are mounted on the helicopter's body. It is a very hard push that keeps the main rotor turning because of all the drag the blades create as they produce the lift. The force exerted by the fuselage on the main rotor is about 600 pounds on an S76, as measured at the tail. To exert this force, the fuselage must brace itself against something to keep from turning in the opposite direction. These are the same choices that you would have as you stand on ice and try to push a car.
Simply said, the tail rotor develops the force that the fuselage braces against as it pushes the main rotor around.