I just though I'd throw my 2 cents in here. Heel toe is the correct term, but wrong concept. As far as I can tell, it actually came from some much older (maybe even the orignal) Ferrari race cars that had the brake pedal on the right and gas in the middle. If you visualize that, heel toeing makes a lot more sense (foot like this / toe on brake, heel on gas).

Heel toeing and double clutching are two entirely different things, but they can be used together. Heel toe was already described pretty well. Use the left flat of your foot on the brake and the right heel on the gas. It takes a little practice but it becomes very natural after a while. You would use this to slow your car and accelerate your engine at the same time. Imaging pushing your car through a corner at it's absolute limit. You enter the car in 4th gear, you want to leave in 2nd. Coming into the turn, you would brake heavily to slow the car, you would turn in, apex, and then accelerate out. Most of the time for an apex, you need both hands, so that's not a good time to shift (especially if you don't have power steering) and if you wait till after the turn, your going to loose speed as you shift, so you want to shift before the turn. The reason you rev match is to make the transition as smooth as possible. Remeber, your cornering at your cars absolute limit, all four tires screaming, starting to loose control limit. If you had the balance of the car upset by the engine as it surges to match rev's, your probably going coming out of that turn backwards. You stand on the brakes and clutch, slide the car out of gear and into the one you want (4th to 2nd) and slide your foot over to rev the engine up. Once your rev's match your speed in 2nd, let the clutch out and your most likely also starting to turn in to the corner. As you come out, you just push the gas down and acclerate out.

For double clutching you would do the same thing, except you'd have to slide the shifter from 4th into neutral, let out the clutch, rev the engine to match then, re-engage the clutch and then slide into 2nd and let the clutch out. I need to get to class right now, but if your interested, I'll explain why when I get back. With modern transmitions, this isn't really nessicary. Seeya.