What is the primary reason for engaging the inertial reel lock during a pilot incapacitation drill?

Engaging the inertial reel lock during a pilot incapacitation drill is crucial to prevent the pilot from falling into the flight controls. This safety measure ensures that if a pilot becomes incapacitated, their body remains securely in place, minimizing the risk of unintentional inputs to the aircraft's controls. By locking the inertial reel, any sudden movements or gravitational forces that could cause the pilot to slump forward are mitigated. This helps maintain the control of the aircraft and allows other crew members to respond effectively to the situation, ensuring the safety of all on board.