This is a good example of Dui-chagi. I can tell you several points of this kick.

First: The kicking foot moves straight way. Focus on the left foot of exampler, and you can see it moving straightly like a thrusting a sword. It is most important of feature of Dui-chagi. Compare this movie with this bad examples. (1), (2), (3) . The (3) is quite corrected one, but still you can see his knee of the kicking leg.

Second: His turning and kicking is not divided. I mean, kicking is a simple movement. Usually, many practitioners kicks in two steps. (1) turn back, and (2) kick. In training, this kind of step-by-step trainig is important and necessary for exact pose and movement. But when the technique is achieved it should be one movement.