When first learning, new drivers are trained to brake with your right foot only. This is to avoid accidentally pressing both pedals in a panic stop situation and also leaves the left foot available to disengage the clutch if necessary.
Racing drivers, especially in rally racing, use left foot braking so that they can shift the weight of the car forward to help in corners without having to come all the way off the gas. How much faster is this technique compared to conventional right foot braking?
In this awesome demonstration, both techniques are compared on a snowy loop. The right foot technique saves about a second on this small loop and also looks much smoother. It is not an easy thing to retrain your left foot to do double duty, but if you have a closed course to practice on, it could shave seconds off your lap times.