The arm drag is more than a technique. It's a principle. If you start to think of the arm drag as a concept, you can begin to see the arm drag position in a variety of scenarios, which will allow you to see new paths to your opponent's back. The goal of an arm drag is simple: maneuver your opponent's arm into a position where he can't set an underhook or an overhook to block you from taking his back. When his arm is across his body in any way, the opportunity to swoop into back mount is there. This may seem silly when viewed in a simplified format like this, but I was blind to this concept for years, and I see many people missing huge opportunities because they are not aware that the back is open.
If your opponent does not have an overhook or an underhook, his back is vulnerable. If he is squared up facing you, you can execute an arm drag and take his back. If he is escaping side control without an underhook or an overhook, you can take his back. If you are fighting to finish an arm bar, you can take his back. If he is defending your kimura by grabbing his belt, you can take his back. If you are in mount, you can take his back.
The video that I shot to illustrate the arm drag principle looks at the last scenario. When you are in mount, it can be all too easy to focus only on setting chokes and arm locks, but you can use your superior positioning to force his arm out of posture and into an arm drag position. I learned this particular technique from Sonny Achille, my head instructor, and it has served me well ever since. It takes some practice, and it requires you to develop some chest coordination (weird, right?), but once you can start thinking of your chest as another hand, you can use it to defeat his posture while using your hands for other tasks. It's sneaky, and it can be very difficult to defend once you've weakened his posture. If you are struggling to complete the transition, evaluate your chest positioning. Ideally, you should be using your sternum to manipulate your opponent's arm. If his arm is too low on your chest, toward your stomach, you will not have the dexterity and control to complete the attack. Check out the video: