Just remember - airfoil drag is equivalent to torque. An unloaded engine creates NO torque. It's only when the prop is biting into the air that torque is created, and at a constant power setting (fixed throttle), more torque => less RPM, and less torque => more rpm.
The whole idea behind a constant speed prop is to control pitch to maintain a constant torque load on the engine, and that results in constant RPM.