All antilock brake systems control tire slip by monitoring the relative deceleration rates of the wheels during braking. If one wheel starts to slow at a faster rate than the others, or at a faster rate than that which is programmed into the antilock control module, it indicates the wheel is starting to slip and is in danger of breaking traction and locking up. The ABS system responds by momentarily reducing hydraulic pressure to the brake on the affected wheel or wheels. Electrically operated solenoid valves are used to hold, release and reapply hydraulic pressure to the brakes. This produces a pulsating effect, which can usually be felt in the brake pedal during hard braking. You may also hear a buzzing or chattering noise from the ABS hydraulic unit. The rapid modulation of brake pressure in the brake circuit reduces the braking load on the slipping wheel and allows it to regain traction, thus preventing lockup. It is the same as pumping the brakes, except that the ABS system does it automatically for each brake circuit, and at speeds that would be humanly impossible, up to dozens of times per second depending on the system (some are faster than others). Once the rate of deceleration for the affected wheel comes back in line with the others, normal braking function and pressure resume, and antilock reverts to a passive mode.