As I understand it, heel and toe is good for maintaining speed through curves. Its done in pro-racing. At times the difference bwtween the guy who spares the car by clutching out and just braking, and the guy who keeps the power on and brakes, is the difference between first and second. Its a racing-thing, and really has no practical application for a normal driver, as far as I'm concerned.
You'll see it done, also, in rally driving, to "provoke" the rear end of a car to step out going around curves on gravel and mud. Stepping on the brakes, not hard, will make the rear wheels lock first. That, combined with turning the wheel and a lose surface, will cause oversteer. Again, not something you wanna be doing to a car that doesn't come with mechanics and spare parts.
Now, double-clutching, as I understand it and please correct me if I'm wrong, is not something newer cars need to have done to them, because they have syncromesh gear boxes. Used to be you matched the revs of the engine with the revs of the wheels, in order to lose as little power as possible, when changing gears. Still used be some cars in racing today because they use straight-cut gears.
I may be talking out of my ass, so get a second opinion...
