This page provides free video
instruction on how to perform a shin kick. In Taekwondo, you
are usually taught to strike an opponent with your instep or
the top of your foot while doing a roundhouse kick. In
contrast, if you use a shin kick, you will hit an opponent
with the dense bone of your shin (versus your instep or the
top of the foot). A shin kick is often used in Kickboxing and Muay Thai due to its power and ability to knock opponents off their feet.
Therefore a Taekwondo shin kick is very effective in
self-defense situations (versus sparring competitions where
you need accuracy and reach).