We call it a "slip kick" in our dojo, but as the comments in this thread indicate, you could call it a lot of things. It's popular in competitive point fighting, because it can cover a lot of distance and score a quick touch for a point. You can throw them out like a jab as part of your combinations, which tends to work fairly well if you get your opponent concerned about your kicks. That said, the mechanics of a slip kick can still benefit you at higher levels of contact, because you are developing that supporting-leg push. I've broken ribs with slip sidekicks, because that push off the supporting leg moves your whole bodyweight into the kick. It's also helped me lay into people with roundhouse kicks as they come in, too. To be honest, I only throw hook kicks when I'm goofing off.
