The flywheel is the interface between the clutch system and the engine. When your foot is off the clutch the clutch disc presses against the flywheel, locking the engine to the transmission input shaft.
When the clutch pedal is pressed the clutch is released from the spinning engine and the transmission disengaged. It's the friction between flywheels and the clutch plate that determines the amount of force that the clutch can hold. Oil, grooves or cracks on the flywheel will prevent enough friction between the flywheel and the clutch plate and cause the gears to 'slip'. If that's the case then it's time for a new flywheel. Show Less