Anti-lock brakes (sometimes reffered to as ABS) are a computer-controlled component on a vehicle's braking system. It is designed to detect when wheels are about to lock up and reduce the braking power incrementally to prevent a lock-up from occurring. Sometimes the ABS is part of the same computer system as the traction control system.
Anti-lock brake systems typically know when a wheel is locked up by reading the wheel speed sensor. When a locked-up wheel is detected, the ABS pump can lower pressure on that wheel (or axle on more simple systems) to let the brakes go a little, usually resulting in the wheel rotating again. With each attempt to make the wheel less locked up, the computer will assess the situation again and pump more pressure out of the appropriate brake lines. When the wheel stops locking up, the pressure will be allowed to occur again.
Why ABS Causes Shudder Under Heavy Braking
When a lock-up occurs, the process to correct is very quick. The computer is constantly checking for a lock-up condition, even if it just finished correcting a lock up. Since the prevention is so effective, multiple lock-ups and corrections can occur in a second, leading to a shudder or vibration feeling when heavy braking occurs.