4MOTION is our permanent four-wheel drive system, offering great grip on the roughest roads and in the worst weather. It constantly monitors driving conditions and reacts to the slightest wheel slip, delivering extra traction to the wheels with the most grip. All this happens in milliseconds, through control units linked to the car's dynamic sensors, engine management system and gearbox. At the heart of the system is an inter-axle differential -the design varies depending on the particular system. The differential's job is to share power between the front and rear axle, sending it where it's most needed. At the same time, the smaller differences in rotational speed when cornering or manoeuvring have to be balanced to stop torsional stresses in the drive train. The differential is designed to work with the car's other active safety systems such as the anti-lock braking system (ABS) and the electronic stability programme (ESP).
Article source: www.volkswagen.co.uk