I like to think of the curl kick as a distant cousin to the backfist. The backfist is designed, in part, to sneak around a partner’s frontal guard – to get a strike to the side of the head when someone is doing a good job of keeping their hands up. The curl kick is designed, in part, to sneak around a partner’s leg – to get in a strike to the groin when someone is doing a good job of closing the straight path of a groin kick.
My thoughts on the kick shouldn’t limit people’s experimenting with it though, I’ve seen it used to kick to the head, the knees and the ankle.
Click on the photo above or here to see a video.