Great video carl. Does the upper arm basically stop when the elbow starts to flex?
Sorry. I missed this till now. If you were asking me, I am fine with other people answering. But since nobody did answer, this is what I will say:
1) I think you are overthinking this. Do a few hundred a day, in front of a mirror so you can see your form while doing the shadow stroke and over time, you will move the stroke to increasingly more efficient technique.
2) The movement is a complex synergy of legs, hips, torso, upper arm, lower arm, wrist. To me, what you are showing looks stiff which may be because you are trying to isolate the arm. But if the idea is synergy with all parts working together, if the upper arm stops when the lower arm starts, then you may as well only use the lower arm because, if one stops when the other starts, THEN THEY ARE NOT WORKING IN SYNERGY (together). For the arm movement to create the fastest racket speed, both would need to be moving at the same time. That being said, there are times when you may take a shot with only the upper arm, with only the lower arm, or with almost no arm movement and only the movement that results from legs, hips core rotation.....So, there is not one answer to your question. But if you are asking for the technique for fastest racket speed then everything (legs, hips, core rotation, upper arm, lower arm and wrist whip) would all need to be in motion at the same time.
3) The question "Is that correct" requires a context. Are you getting more use of the elbow joint (lower arm).....yes. Are you getting the upper arm and lower arm to work together.....not really. Are you getting the upper arm and lower arm to work with the rest of your body to help create the muscle memory for the complete complex of synergistic actions that create a powerful and effective stroke......definitely not. BUT YOU ARE GETTING YOUR LOWER ARM (elbow joint) TO MOVE as part of the stroke. So, for the purposes of what you were trying, that may have been what you were looking for.
Practice the shadow stroke facing a mirror and do a few thousand (20-30 min) a day for, say, 4 days this week and 4 days next week (8 days 20-30 min of non stoped shadow strokes) and then film what you are actually doing as your shadow stroke. See if it looks different as you give yourself time to feel the coordination of using everything together for 8 days (over a 14 day period....rest time and time for your nervous system to absorb some of the information).
Hope this is helpful.