I can provide a descriptive image if necessary, but it seems reasonably evident to me. Normally, roundhouse kicks deliver a more lateral blow, generally striking and then pulling the leg back whether impact was made or not. However, if your kick is higher than your target, it's also possible to continue the arc of the kick into a downward strike (or at least a more downward diagonal), possibly evading a person's guard by going over it. The strike as with a regular roundhouse, it's still worth the instep, or the ball of the foot. In Capoeira, it becomes more common for people ducking the roundhouse and the attacker continuing to strike down at the dodging opponent (also because the rules of engagement in Capoeira make fully rotating kicks safer since you don't have to worry as much about being struck in the back).
There's a variant of this that you sometimes see as a more acrobatic move in Capoeira or in Kung Fu-inspired martial arts films where it's done as a jumping technique, making a horizontal rotation to land a spinning kick that's lateral compared to your orientation, but vertical in respect to the target. Jackie Chan seemed fond of using it to strike an opponent kneeling or lying on the ground to finish them off. You can see it (briefly) at about 1:34 of https://youtu.be/4YjGxNDPLJA.