Left foot braking is just that - brake with your left foot, not your right.

As for the understeer issue - like everyone else said, brake harder before the turn. What you do now is costing you speed. If you brake at the same point, but harder, you'll probably find that in doing so you'll be going pretty slow . This is good because it shows that you can now get on the brakes later. Then you modulate the throttle until you're around the apex of the corner (ie, you get on the gas, but not full). As you get around the apex, you go full throttle, while at the same time unwinding the steering wheel.

What you're doing now is going full throttle before you unwind the steering wheel. That requires too much of your traction budget (remember when you accelerate you have less traction for turning) and the tires slide. So if the tires are sliding, you're basically sitting there waiting since no power is getting to the ground. And waiting is slow .

In tight turns, many times there's nothing you can do about it but go slow around them. Try to force it and you lose grip. I took an Evolution autocross phase 1 school and there's a skidpad at one point in the course. Going slow to go fast was really hard for me to learn, but every time I got on the gas too early and pushed, my times just died.

I just went to an autocross last weekend and used all of my runs to test out different techniques etc. to see what would happen. Didn't care about times, I just wanted to see if I could feel what was happening and what I did to cause it. So I suggest at the next autocross, try braking harder at the same point, then moving that brake point up when you feel you braked too early. Also, you are probably early-apexing to get around the corner the way you do so try this: when you think it's time to turn in, count to one and a half, then turn in. And if you squeeze that throttle down before you start unwinding, it's too early.