The anti-lock braking system prevents the wheels from locking up during an emergency stop or when braking on a slippery surface, ensuring that drivers are still able to steer the vehicle. When the wheels lock up, they are no longer able to transmit cornering forces, meaning that the driver loses control of the vehicle. To prevent this from happening, the ABS control unit uses wheel speed sensors to monitor the rotational speed of all of the vehicle's wheels. If there is a risk of a wheel locking up, a solenoid valve in the anti-lock braking system's central control unit reduces the brake pressure applied at the wheel in question until it starts to rotate freely again. The pressure is subsequently increased to the lock-up threshold again. The vehicle remains stable and controllable. On the latest anti-lock braking systems, as fitted on all Volkswagen models, this process is repeated several times a second. The driver knows when the anti-lock braking system is at work because the brake pedal pulsates slightly. Within the anti-lock braking system's scope of operation, the vehicle can be steered without difficulty, even when braking at full power, allowing the driver to take evasive action to avoid colliding with any obstacles in the road ahead. The "off-road" ABS function featured in the Touareg is a special algorithm programmed into the ABS control unit which is activated automatically only when driving in extreme off-road terrain (vehicle must be travelling at less than 5 km/h with the centre differential locked, for instance). With "off-road" ABS, the wheels can lock up briefly before the system reduces the brake pressure. This allows time for a small wedge of material from the ground (such as gravel or sand) to build up in front of the wheels, which then increases the braking effect. The Touareg remains steerable at all times while stopping distance is significantly shorter.