An electrical sensor is mounted at each wheel to measure wheel and tire rpm. The sensors send alternating or pulsing current signals to the computer. If one wheel slows the sensor signal reduced frequency and the computer activities. The hydraulic valves to reduce pressure to that wheel brake assembly. This keeps the tire from skidding.
If a cars tire were to lock up and slide, the car would Not Stop efficiently. A car stops the fastest when the tires are almost ready to skid. The skid control system can detect when the wheel speed drops rapidly (ready to skid) the control unit then send control pulses to the actuator. The activator then cycles the brakes ON and OFF very quickly for a controlled stop. Since exact skid control system vary, always refer to a shop manual for more details of system operation.