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.
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")