Typically this is used in a car that has understeer normally, but has trailing throttle oversteer (i.e. tends to rotate or spin when you let off the gas in a corner). Most FWD cars tend to be this way, and some RWD cars.
By using left foot braking, you tend to slow into the corner, leaving one foot slightly on the brake to drag the rears and throw weight forward, and at the same time get on teh throttle early. By staying on the throttle through the corner, and modulating the brake with your left foot, you can mitigate teh throttle on understeer by using the trail braking oversteer and get a nice, neutral line that can change easily from a little more of one or the other depending on need in a particualr corner like a decreasing radius).
In a FWD car your brakes really aren't working against the throttle, so it really can tighten up the corner without going into a spin, and allow you to get on the gas earlier without going wide in the corner. Done right, you can nearly pivot a FWD car on the throttle and go through a corner much faster.
-- Chris V ------------------------------------------------------------------------
