Left foot braking is a key skill in racing and performance driving, but it can also make you a safer driver on the street, reducing reaction times in emergency stops and (with additional car control/weight transfer instruction) help you maintain or regain control of your vehicle, particularly when driving on a loose or slippery surface (New England winters, anyone?). In this excellent video the guys at Team O'Neil Rally School giving an explanation of how to use the left foot braking technique.

Disclaimer: I am not responsible for those of you who launch yourselves through the windshield while first attempting left foot braking.