One thing not mentioned yet is that the rotor system is run at a fixed RPM, chosen as a compromise between the most efficient angle of attack and enough RPM to give you a chance to get into autorotation if the engine stops. Newer machines might take off at one RPM setting, and cruise at a lower setting for efficiency.
To keep the RPM constant with varying pitch (and load) settings will require the engine to put out differing amounts of power.
With a turbine, the throttle is usually set to full open, allowing the governor to use any amount of engine RPM (N1)up to its design maximum. In older piston engines, it is up to the pilot to work the throttle to keep the rotor RPM in limits. Usually the engine RPM (ERPM) and the rotor RPM (RRPM) needles are on the same tacho, so we always want to see the needles matched.