Plain English Meaning
A part that lets the pilot disconnect the hydraulic power assist from the controls so the controls can still be moved by hand if the hydraulic system stops working.
Full Definition
A mechanical component in a hydromechanical flight control system that allows the pilot to mechanically separate the cockpit control input from the powered (hydraulic) actuator, so the control surface can be moved by direct mechanical means if the powered system fails.
Context Anchor
Seen in diagrams and descriptions of hydromechanical flight control systems, where cockpit controls are connected to powered units that move the control surfaces.