Why does he need to "pull" when he's already stepping/pivoting at a 45deg angle? He can already land that hook, which looks like a Boxing check hook.
He doesn't need to pull. The problem is that the pull makes things worse and not better. My assumption about it is that he saw this technique in another system and didn't bother to ask what makes it work. The general rule with pulling an arm is to pull it across your opponents body or to the side of your opponent.. The other general rule is that a pull has to be disruptive. It has to cause a person to lose balance, in the demo version it does. It explains a concept. In the assumption that he makes.
If you notice in the first 4 seconds of the video he is actually walking into his opponents punch. All his opponent has to do (in lack of a better description) is a one inch punch. Here straight beats circular. When his partner tries, he makes the same mistake as well., He's literally running into the fist. At 0:36, DK Yoo seems to realize his technique isn't solid and that it has some serious flaws in it. And he still gets it wrong.
If I land this technique as what I think it's true intention is, then it's lights out. My opponent isn't going to get up and I'm not bragging about that. If someone did the same to me, it's the same result. No matter how tough I think I am the technique that he's showing, when done correctly is dangerous. Boxer use a similar technique but they don't target the vulnerable spot because of the rules of boxing.
The concept is solid when it's done correct. Here's the general concept along with variation of it. Notice how he's not pulling the hand down and towards him. He's pulling it across the body on either side.
I teach this as "addressing the lead hand" The lead hand is the hand that pops you for getting to close, so you always want to address the lead hand first. Most people use their weak hand as the lead hand, so it's quick but not powerful, but it's powerful enough to disrupt an attack. But if you ignore someone who fights with power hand forward, then they will make you pay a price for not "addressing the lead hand" which in this case it the pull.
My free kung fu secret to the group. Interfere with the lead hand correctly and the rear hand will have trouble firing.