Dui-Chagi is the most powerful kick in TKD, since it includes the force from turning back with from spreading the kicking leg. And addition to that, it is fast enough. So it is used in competition as a major technique, like the scene of the right side.

But as much as it is powerful and speedy, it is difficult to practice, and more to skilled in exact motion. For beginners, it is almost impossible to mimic its motion.

However, you can find good chance to learn this kick with friendly explanation, because most masters are already skilled in this techique, so they come to think it is easy for every one as easy for them.

I recommend you follow the steps I suggest here to get this good technique if you are a beginner.

The first thing you have to know is the whole step of Dui-chagi. The following image shows it step by step. First ready, second turn back, third lift your kicking leg high under your trunk, forth kick back, fifth drop it steping forth(backward in your pose), and seventh get ready again.

Let me tell you some points of each step, now. Let me begin with the second step as the first is an usual ready pose.

In the second pose you should push your trunk to the direction of your kicking. I mean you should turn your body pushing it. You should not turn fixed on the primary spot. Loot at the second pose and compare it with the image of the right side. I marked the centroid of two pose in the images so that you can compare both easily. The centroid of the right image is between his two feet, while the first image has it on his one foot. If you don't push your trunk like that, your kick will be slower. It is easy to say why. Loot at the third step. You have to stand on one leg finally. So you have to move your centroid after you've turned back any way, adding one more step. It will dicrease your speed in Dui-chagi.

The third and forth steps are the critical parts of the whole process. So I've lightened them distinctively. Firtly, give your attention on his movement of head. He see his target over his left shoulder in third step while he see it over the right shoulder in forth. It is important that he turned his head suddenly, not with the spin of his trunk. It is good to keep your sight on the target all the way, or as long as possible. And it is why he doesn't feel so giddy in spite of his many times of Dui-chagi. Remember the organ that controls your balance is in your ears. The longer you turn your head the diggier you may feel.

Since the third and forth steps are critical parts it is difficult and important to get Dui-chagi. The other point of those steps is that you should not turn back your trunk during a kick. Let's see again the whole step. In forth, he looks back at his target over his right shoulder. It means he didn't turn his upper body back. Of course you can see that he turned it slightly, but he didn't intend it. In addition to that, give your attention on the fifth pose. He is still standing back to the target after his kick. If he's turned his body during the kick he cannot stank like that in that step.

Then let me tell you why you must not turn your trunk back during the kick. An essence of Dui-chagi is to kick along straight way. Otherwise your kick would be shorter and round. For your development your basic motion should be distinctive.(Refer to Ch.55 of PPT) If you cannot but kick round, you'd better try Hwai Chuk(Reverse kick). But it is difficult to do it actually even after full understanding. So the following practice is required.

This practice focuses on only the third and forth steps. You should stand straight up and lift a leg and kick it exactly backward not looking back. Then, what is the points of this practice?

The first point of this practice is to kick exactly back, which is most important. If you practice whole process of Dui-chagi only, as far as I've experienced, you cannot avoid turning your trunk back(particularly, your hip) during kick(3rd and 4th steps). It is the matter of habit rather more than of understanding or intention. So we need to separate only this part from the whole. In this practice it would be easier not to turn your trunk back during the kick. You should always intend to kick straight, and investige how to do it. You will find it still difficult.

The next point is not to see back. If you see back you will come to turn your body back. And, you have another reason not to see back. In actual Dui-chagi you come to meet a moment when you cannot see your target. You should be able to control your kick in that moment too. Thus, you should be able to kick exactly back, exactly to your target not seeing in exercise. After thousands times of repetition in right practice, you will be able to make exact Dui-chagi.