Cadence braking is a technique used to stop a car or other vehicle more quickly on a slippery surface. It would normally be used to effect an emergency stop where traction is limited, though for use in an emergency requires a presence of mind that the situation itself might preclude.
Maximum braking force is obtained when there is approximately 11% slippage between the braked wheel's rotational speed and the road surface - at this point rolling resistance is maximised, and there is a small additional contribution from sliding friction - beyond this amount of slippage, rolling resistance diminishes rapidly and sliding friction alone slows the vehicle. Due to local heating and melting of the tyres, the sliding friction can be very low.
Cadence braking involves pumping the brake pedal fairly rapidly but deliberately, to make the wheels lock and unlock. Because a locked wheel creates a smaller braking force than one which is just on the point of locking, this technique will cause the vehicle to stop more quickly, because the point of optimum braking is passed through multiple times. In addition, by avoiding a total lock-up, steering control can be retained, at least in part. While cadence braking is effective on most surfaces, it is less effective than keeping the wheel continually at the optimum braking point. However, this is almost impossible to do manually, and so in practice cadence braking is a better technique for manual control. Note that the principle of cadence braking is automated in anti-lock brakes (ABS). When ABS is present the best emergency stop will be obtained by simply pressing hard on the brakes, forcing the ABS to perform.
Cadence braking will not help on extremely slippery surfaces such as ice (in theory it would, but in practice the ice can be so slippery that it makes little difference -- a winter tyre would make more difference), or on very loose surfaces, where a quicker stop can be achieved by simply locking the wheels - a wedge of loose material will build up ahead of the wheels and create a substantial braking force. However for most surfaces encountered, cadence braking will almost always stop a vehicle more quickly than either steady braking or locking the wheels.