Thank you for posting the video.
I wonder. . .
After taking the time to make the video and think/feel through the process, do you have anything different to add?
Have you made any new distinctions between "activate," "raise," and "use" as they relate to the shoulder, or even about what exactly someone may mean when they say "shoulder"?
If this is not "internal," how does it fit into "internal training"?
When you work on this shoulder/arm raise, you start off using the muscle groups I am referring to. Over time, wether you do the arm raise, or hold it in a static position, you will find that you no longer activate (or fatigue) the area under the arm, it will move to the front of the body from the arm pit, then it will move progressively lower and lower., until you feel it in the upper abs, then the lower abs/hips to the area generally associated with the dantien (though this requires doing other things before it gets to this point). This is why I said that when you initially try to do it this way you are still using "local" muscle, just different ones.
Now when you then raise your arm, you won't feel a sequential activation of each muscle group that you previously learned to activate (and later relax). Are they actually activating? Beats me, but you tend not to feel them activate, so perceptually I think its fair to say that you don't feel it, nor do you tend to feel it if you put you hands on someone who can do it.
When I refer to activate or raise in reference to the shoulder, its much more like what I am showing in 1-2, you feel a tension/load there, but nothing else in the muscles associated with the inside/underside. As I stated earlier, this is because most people only really use one side of the body. You will see the same sort of thing in the legs (less of a load in the quads, as the inner thigh and the area below the buttocks take it up), as similar exercises may be performed with the hips.
Using both sides lessens the use of the shoulder. Now yes it feels like no shoulder is used because the load is not mostly taken up there. To me in the end, if it is actually used or isn't used is is not relevant, because the perception to the person raising it is that it is not.