A clutch slave cylinder is a component of a hydraulic clutch system on a vehicle with a manual transmission. The slave cylinder is a small, cylindrical pump that remotely receives fluid pressure from the clutch master cylinder.
When you press on the clutch pedal, the master cylinder forces hydraulic fluid through a thin tube to the slave cylinder. The slave cylinder, in turn, applies pressure to the clutch pressure plate to cause it to disengage the transmission from the engine. When you release the pedal, the slave cylinder forces the fluid back to the master cylinder and raises the pedal to its normal position.