Most martial artists I've met think this is one of their weakest kicks when it could actually be one of their most powerful weapons in their arsenal of kicks, punches, and other strikes. If you're the average martial artist, you throw your rear-leg roundhouse kick from a front stance snapping your leg across your body depending on the weight of your leg and the snap of your knee for power. Power is a combination of mass can be used in this example) and speed. Kinetic energy is half mass times speed squared. The more mass, or the more of your body you put into your kick, and the greater the speed of your kick the more powerful your kick will be.