I pointed out his problem I stated here, and he tried to correct it. It is not easy to accept someone's advice and change his technique in short time. But he did, faster than I had expected.

But he is not perfect yet. He draw back his foot after he finished his kick. It is unnatural.

Of course, you will be taught to draw back your foot right away if you finished kick, and you should try to do so. But after you built good basic motion, you should pursue more ultimate thing: the attack and defence, and not the motion itself. And for that, you shoud be ready as quickly as possible, for the next attack or defense.

Remember, the way of Saram(Man) in Taekwondo is to return immediately to a settled pose which is the core of each motion of yours in perfect balance despite endless changes and motions, or to make your position firm staying in your complete balance and stability.(PPT, Ch.12)