system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: What is the "heel and toe" driving technique and why is it useful?
gpt
It's a technique for downshifting smoothly, and is useful when you need to downshift mid-corner and do not want to upset the balance of the car and risk losing drive-wheel grip from a jerky shift. You see it a lot in "decreasing radius" corners, which means they get sharper as the corner goes on requiring you to slow down.

You use your big toe to press the brake, your left foot to press the clutch, and downshift. Before releasing the clutch, you rotate your foot to tap the throttle (with the other side of your foot/pinky toe) while still braking. This revs the engine a bit, so when the clutch is released, the revs match the road speed and the change is smooth.

You don't actually use your heel most of the time, it's just the term that's been applied, because "big toe and side of foot" doesn't roll off the tongue quite as well. Some cars with a throttle hinged from the bottom (like Nissan Z cars, or an Acura TL) do let you use the heel. Top-hinge throttles like most cars have require the side of your foot or little toe to be used because your heel won't reach.