When you depress the clutch pedal, the force of your press is transferred hydraulically to the clutch: the master cylinder attached to the clutch pedal builds up pressure, sends it along your clutch line, and the pressure pushes the piston in the clutch slave cylinder out against the clutch release fork inside the transmission. When all this happens correctly, pushing the pedal down pushes against the spring-loaded clutch pressure plate and the clutch is released.

When this DOESN'T happen correctly, like if there's a leak somewhere, pressing the clutch pedal won't release the clutch. This most commonly manifests as the car not wanting to go in to gear with the clutch pedal in when the engine is running. Because of its exposure to heat and the elements, the most common clutch hydraulic failure point is usually the slave cylinder.

This slave cylinder is made by FTE, the Original Equipment Manufacturer for this particular Mini clutch slave. The only difference between this one and a Genuine Mini clutch slave cylinder is the Mini box.