Heel and toe "downshifting" is pushing on the brake pedal with your right foot, but as you are between gears in the downshift, you blip the throttle (while still pushing the brake) to bring the revs up before letting the clutch out. This is done in racing so you can get maximum braking and not have the wheels do engine braking when the clutch is let out. Say that you don't do this. You're at maximum braking that the tires will hold (just before skidding). If you downshift and don't blip the throttle, the engine's going slower than the tranny when the clutch gets let out and the engine brakes the tires some more. You skid. I don't mean that you necessarily slide out of control, but you skid.

There are 3 techniques to heel and toe brake.

1: Heel of the right foot on the brake and toe on the gas.

2: Toe of the right foot on the brake and heel on the gas.

3: Right foot on the brake but slightly off to the right and then blip the gas by sort of twisting your foot down. This requires pedals to be very close together.