The impeller clutch piston (3) receives oil from the impeller clutch solenoid valve. As oil from the impeller clutch solenoid valve increases, the piston engages the clutch pack. The friction between the clutch pack discs and plates locks the impeller to rotate with the clutch housing. As oil pressure on the piston decreases, the friction decreases and the impeller slips (turns slower). This slip forces less oil to the turbine resulting in decreased torque at the output shaft.