What happens to the CSU when the propeller overspeeds its target RPM?

When the propeller overspeeds its target RPM, the critical function of the Constant Speed Unit (CSU) is to adjust the propeller blade pitch to maintain an appropriate engine RPM. In order to achieve this, the CSU allows oil to flow into the propeller hub, which changes the pitch of the blades to a higher angle. This increase in pitch creates more drag on the propeller, which in turn reduces the RPM and stabilizes the speed of the engine.

It's important to understand that the CSU is designed to optimize efficiency and performance by managing propeller blade angles in accordance with the engine's power output and operational requirements. By adjusting the pitch, the CSU ensures that the propeller remains within safe operational limits, preventing potential damage from overspeeding and maintaining overall flight safety.

The other options posed do not accurately describe the primary mechanism by which the CSU operates in response to an overspeed condition, as they do not involve the critical adjustment of propeller pitch necessary to manage RPM effectively.