The heel-and-toe gearchange is an advanced driving technique which
allows for smoother driving and braking. And smoother braking
means you can brake harder and faster which ultimately results
in faster lap times. Heel-and-toeing is done so that consistent
pressure can be applied to the brake pedal during downshifting.
Heel-and-toeing is done by keeping the top of your foot on the
brake during a downshift and blipping the accelerator with the
right side of the foot to bring up the engine rpms as the downshift
is made.
A good way to practice a heel-and-toe gearchange is to blip the
throttle during a downshift to match the revs of the new gear
without braking. Slowly, and as your skills improve, introduce
braking into the mix.