Dui-Chagi2 : essence
Dui-chagi(View from Behind)
Dui-chagi(View from Forward)
The first thing you have to know is how it can produce its big power with good speed. Its speed is closely connected to its power; It means, because it is quick it can be powerful. Generally speaking, speed is an important factor of power.
Look at the left image. The lines represent the orbit of kicking foot from the upside.
The blue line is not the orbit of Dui-chagi but that of Hwae-chuk. The red line is for Dui-chagi. The first thing I have to emphasize is that the main movement of Dui-chagi is the straight one. Look at Dui-chagi(View from Forward) . And compare it with Hwai-Chuk(View from Forward)
For your better understanding of what I emphasize, I have to notice that everyone recognize the kicker spins his or her body for Dui-chagi. Yes, it is right. But you should not attached to the visible thing too much. What you see easiy is not the essence of technique nor of Taekwondo. If you intend to spin as you saw him or her spinning in their kick, you will fail to make good kick. The essential part of technique is in your mind, as TAEKWONDO is the way of mind(3JKY 9-2). Concerned with Dui-chagi, you have to understand that you should intend to make straight kick backward for a good Dui-chagi.
As Dui-chagi moves straight it can touch your opponent quickly. But as far as you intend to kick backward, although you intend to make straight one, you cannot but spin back. That spinning makes big power. And the spinning back is natural, so it is not easy to control: sometimes, or more often than that with beginners, its spinning overwhelms your intention of straight back kick. So most beginners come to kick like the right image(The blue one is good, while the red bad but prevailed).
If you belong to the prototype of failure I've said, you must be spinning back too much. The first source of this is in your mind. In most cases, you want to see your target while kicking, so you must have turned your shoulder back, so that you can see the target before your kick. You should not do so. Then, how? You should train yourself to be able to kick before your sight at the target. Look at the image below, particularly the brightened part. The kicker didn't turned his shoulder though he turned his head back to see.
Of this movie( Dui-chagi(View from Behind) ) the first example is not so correct as the next two ones. And pay attention to the kicker's pose when his kick touch the target. His hip, his leg and the center of the target must be on a straight line. Otherwise, the kickers weight cannot be used for the power.
Of this motion( Dui-chagi(View from Side)) you have to pay attention to the kickers step. I mean, he make one step forward after kick. In other words, he kicks during his kick. Another to say, he corrupts his balance before kick, utilizing its power, and comes to step forward naturally. When the balance gets perfect, that is, it accumulates most possibility in itself, it can be an important source of power.(PPT, Ch22) You can also see it on the below image.