Yeah you asked me about how I pull someone's arm straight for when they grab me from behind. So in the first clip that you show (below) That turning that he does. I don't turn behind the person. I step away from the person. In the clip he only turns. I turned and steped away from him because my intention was not to get behind the person. When you turn and step away that arm will extend. He's also using a narrow stance where mine was wider and lower. In the clip there's no way to pull the person as I described if you are that close. But if you step back you get that the opportunity to pull down and and at a 45 degree angle.. The reason you have to pull down first is because your opponent will have a higher stance. You want him to bend over a little more like what you see here. You want him in that awkward position because it's difficult to maintain a good structure when you pull downward at at 45 degree angle then switch it up on him by then pulling horizontal. Think of it as you trying to get him low enough to the ground where you can drag his face across the ground.
The awkward position and the twisted arm will keep him from being able to use that arm to resist. You'll know when you get that arm locked because you'll feel tension as if you were to continue something will pop out or break. So keep that in mind if you are using this with a training partners. Things like this don't just throw, but they also damage the joints in process when the person tries to resist.