Final answer:
A mechanical locking mechanism locks the Power Control Levers (PCLs) during rotor brake operation to prevent inadvertent throttle changes.
Step-by-step explanation:
During rotor brake operation, a mechanical locking mechanism typically locks the Power Control Levers (PCLs) in the idle position. This mechanism ensures that the PCLs cannot be moved out of idle while the rotor brake is engaged, preventing any inadvertent increase in engine power that could lead to damage or unsafe conditions. The exact locking mechanism can vary depending on the aircraft's design, but it usually involves a pin or latch that engages when the brake is applied, thereby physically preventing the PCLs from moving.