The portion of the tank valve damaged by over-tightening the valve knob upon shutoff is:

The valve seat is the component of the tank valve where the sealing occurs between the valve and the tank. When the valve knob is over-tightened upon shutoff, it can compress or deform the valve seat beyond its intended resistance, leading to leakage or failure. The valve seat must maintain a precise fit to ensure that it effectively seals under pressure, so excessive force can damage its structure and compromise the integrity of the valve. This reinforces the importance of following proper procedures and torque specifications when operating tank valves to prevent damage and ensure safety during diving operations.