On a constant-speed propeller, which has been likened to a car’s automatic transmission, blade pitch is adjusted by a governor, an engine-driven pump that monitors engine rpm and uses oil pressure to vary the pitch of the blades to maintain that rpm, regardless of changes in airspeed or power settings.
At high rpm, the blade pitch is low—taking a smaller bite of the air and decreasing angle of attack—and the prop wants to spin faster. To reduce rpm, the governor moves the blades to high pitch so they increase angle of attack, take bigger bites of the air, and slow the engine down.
If there is a loss of oil pressure in the governor, a constant-speed propeller will go to low or “flat” pitch (knife edge to the airflow), the blades will encounter no air resistance, and the engine will consequently overspeed.
An aerobatic constant-speed propeller has a large counterweight on each blade root. If engine oil pressure to the governor is lost in zero-G or negative-G manuevers, the centrifugal force of the counterweight drives the blade to high pitch—the maximum surface area is presented to the airflow—and the engine “underspeeds,” which prevents any overspeed damage. Throughout an airshow performance, you will hear a howl from the propeller as the blades shift.