Left-foot braking is the technique of using the left foot (as opposed to the more usual right foot) to brake a modern car. It is most commonly used in auto racing.
At its most basic purpose, left-foot braking can be used to decrease the time spent between the right foot moving between the brake and throttle pedals. It can also be used to control the load transfer.
Karts, many formula cars, and some modern road cars (such as the Ferrari Enzo), have no foot-operated clutch, and so allow the driver to use his left foot to brake.
One common race situation that requires left-foot braking is when a racer is cornering under power. If the driver doesn't want to lift off the throttle, and potentially cause a trailing-throttle oversteer situation, left-foot braking can induce a mild oversteer situation, and help the car "tuck," or turn-in better. Mild left-foot braking can also help cure an understeer situation.
In rallying it applies primarily to front wheel drive vehicles. It is closely related to the handbrake turn, but involves locking the rear wheels using the foot brake, which is set up to apply a significant pressure bias to the rear brakes. The vehicle is balanced using engine power by use of the accelerator pedal, operated by the right foot. The left foot is thus brought into play to operate the brake. Rear wheel drive rally vehicles do not use this technique because they can be much more easily turned rapidly by using excess power to the wheels and the use of opposite lock steering.
This technique should not be confused with Heel-and-Toe, another driving technique.