The emergency manual control was a component of Data which enabled him to be shut off. This would typically be due to a malfunction, but could be done at any time.
The control was apparently an element of the secondary neural processing network but drawing power from a protected reserve buss, in the primary cerebral circuit.
The control was in a primary module with attachment points which were plastic pins chemically bonded to the spinal framework. The housing was constructed of a hermetically sealed yurium membrane that was independent of the enclosed environment of the upper torso.
The control was achieved by manual pressure on the control points. There were four control points: for cognitive system activation/deactivation, for primary locomotive control, for manipulator servo control, and for visual sensory input control.
Due to Data being regarded as a sentient being, few people knew of the control's location, although it was detailed in his schematics.
When Data's status as such a being was called into question in 2365, William Riker was tasked with proving Data was the property of Starfleet, despite not believing it personally. He studied Data's schematics and discovered the emergency manual control, which he used to great effect in the courtroom on Starbase 173. (TNG: "The Measure Of A Man")