Almost everyone has this fault in basic kicks. This fault cause an unnecessary sign before his actual kick, so it spoils his attack.

If you see this slow motion of his kick, you will see that he make a slight step with his fore foot(left foot), and then continue his kick with his right foot.

Remember, "Concerned with motion, it is following the way of Haneul to connect only necessary movements with no excrescent one. "TAEKWONDO is very simple. It is in one, and in nothing more than one."(Ch.55) With excrescent movements added, it is not one yet many. And it is many in disharmony."(3JKY, Ch.3-6)