Thanks for the video and explanation Hunter! This may be totally unrelated and off-topic, but the part where your body comes out to counterbalance the arm raise reminds me a little bit of this video --
(Footnote: I have no idea whether the video here fits the description of internal movement, but it definitely shows some very interesting ways to move the body for sure.)
Back to slightly on topic, this discussion about arm raises makes me think about standing post, particularly of the "hug the tree" variation. I was told by one instructor that one of the goals is to be able to do it effortlessly without any stress to the body/muscles through relaxation, and various instructors have talked explicitly about starting off in a standing position with the arms down, relax, breathe, and eventually the arms will start "floating". I've felt a little bit of this myself in my standing, though usually my arms float only about a few inches upward from their resting position (and I often wonder then whether I'm even Doing It Right or not).
My current thought is that the tree-hugging standing post done properly is nothing more than a regular standing post with the arms floating all the way up to that position.
(Which makes me wonder whether it's helpful/unhelpful/incorrect/dangerous to start from the "hug the tree" position and try to relax from there, since someone without this experience/ability would necessarily be using muscle to hold the arms up instead.)