Heel/toe works like this:
"The goal is to downshift without upsetting the balance of the car. ie. matching revs while downshifting.
This requires you to
1) Brake
2) Blip the throttle to match the revs of the engine so that when you are in the lower gear that the rpms are appropriate for that speed, keep in mind you are slowing down while this is happening.
3) Downshift
While braking with the toe/ball of your right foot, you shift push the clutch in. Now blip the throttle with the side of your right foot (heel, for me it is more the side of my right foot) and then slide the gear lever to the lower gear. While disengaging the clutch.
This should produce a seamless downshift, one you can do while entering a corner, or braking hard, and the car should not become unbalanced."
I borrowed this from another forum I'm on, rather than rewriting the same thing myself. Another way to think of it that makes more sense is Ball/Edge downshifting. You use the ball of your right foot on the brake, and the edge on the gas.