In this tutorial, Master Sung teaches the proper posture and execution of a dropkick. This kick has had a few names through the years, downward kick, axe kick, and hammer kick. It is one of the few kicks that is used when your opponent is very close because you bend your knee close to your chest, lean back slightly, and drop your heel. It can also be quickly changed to a push kick if needed.
https://www.youtube.com/watch?v=1pZ8ZG3gu40&t=251s