ABS is the abbreviation of Anti-lock braking system. The system is used to prevent the wheels from locking up in case of skid conditions so as to avoid uncontrolled skidding. The ECU constantly monitors the rotational speed of each wheel. If it detects a wheel rotating slower than the others it reduces hydraulic pressure to the brake at the affected wheel, thus reducing the braking force on that wheel. Conversely, if it detects a wheel running faster than the others, it increases the brake fluid pressure so the braking force is increased, slowing down the wheel. Some anti-lock systems can apply or release braking pressure 15 times per second.