zymotic:
Double clutching and Heel Toe are very useful for driving a car at full potential.
For instance.. come into a corner at high speed, and jam it down a gear (engine brake) this will cause you to instantly loose traction and crash due to a huge weight shift to the front wheels.
Double clutching matches the output shaft in the gearbox to the engine crank speed before putting the car into a lower gear, doing this makes for a smooth transition.
This is achived by: coming into the corner, putting your foot on the clutch and pulling the gear into neutral, releasing the clutch, reving to match, then engage the clutch and lower gear.
Now while you are doing that.. you have no control over the brakes..
This is where Heel toe or Rolling the foot comes into play.
While doing the rev matching you will have your toe on the brake and rev match with your heel. I prefer to just have half my foot on the brake and accelerator pedal, normally I find it easier.. but it's probably a little dangerous too as you can stomp the accelerator instread of the clutch
Ok.. thats my explanation, some points are probably not 100%.. and it's really hard to explain it's just something you get with practice.
First time you see it and try and do it, you go WTF! my feet can't move that fast!