Let's think. Why the attacker should attack the defender by way of the patternized way; Apkubi Arae Makgi and then Apkubi Momtong Jireugi? Don't you think you cannot move that way in actual case?

To say conclusion first, the motions you make at the beginning of the yaksok kyorugi is effecient in actual case. But it is higher level technique than you might expect. This is what you should understand here.

Imagine first some actual case. How would you attack your opponet actually if you have to attack him? Or how he will attack you actually? You or he may punch at the opponent's face. Yes, it is ordinary thing. But this attack has clear limit.

When punched at his face, many people would be hurt easily. But in some cases, particularly when everyone is desperate, this kind of hurt cannot control situation completely. I mean, the attacked still can attack you back right away. And then, it would be more dangerous than before.

The better attack, it it were, would be something that can be so critical that the defender cannot get back in his energy or stemina. I mean, the first attack should knock him down at once. But how? You should punch at his vital point, which is close to you and easy for you. So almost every martial arts recommand you punch at the solar plexus. It is what the attacker do at the beginning of the yaksok kyorugi of Taekwondo.

But there is the right way of attack and defence for every technique, which is called the ways of Samjae. It is to drop first(Haneul), approach(Tang), and then, hit the opponet(Saram). (Refer to Ch.8-2 of SJKY)

Now let's compare two caes. First usual case, and second the better case recommanded in Taekwondo.

This is the usual case. The right side is attacker here. He attacks his opponent suddenly, but the defender come to avoid it slightly, frightened by instinct. It means, it is very difficult to attack the defenders face exactly. Here, focus on that the attacker punches without drop of his body. He just approaches.
This is the better case. The right side(attacker) drops his body first and punches at the opponent's trunk. The defender is frightened and avoids, but he cannot move his trunk much, failling to avoid the sudden attack.

Taekwondo enforce this process with formalized actions, and you should understand the technical factors of Apkubi Seogi.

This is simple but difficult. So you need your master's guide. Refer to building blade.