Heel toe downshift is the technique of downshifting on your car where you match the revs of your engine and transmission and then downshift instantly while braking. What you basically do here is use your right foot to brake, and simultaneously blip the throttle to match revs with the same foot using its heel. You then use the left foot to depress the clutch lever and change the gear. This might sound easy but believe us, it is not, it takes a lot of practice and patience to learn. It's an art that only comes through practice and more practice.
What are its uses you ask. Well for one it allows you to eliminate all the engine jolt and prevents the rear wheels from locking, contributing to cornering stability. It also keeps the rear in check and prevents it from going loose while taking a turn. It is mostly used on racetracks and mountain roads where you encounter many high speed corners but once you learn the technique there is no stopping using it on regular roads as well.