This roundhouse kick is performed with your leading leg.
Starting in your fighting stance.
The idea for a hopping roundhouse is to swap your leading leg for your rear leg. You can perform a little hop to do this, or alternatively just bring your back leg next to it. In this instance I took my rear foot behind my leading foot.
With your rear leg taking over the position of your leading leg, you now perform the roundhouse kick. Here it is shown as a low roundhouse to your attackers leg.