Lately, and I could be wrong, but I’ve been getting the feeling that the tightness in my corners are actually to push open the aperture / keep the roof from collapsing rather than trying to close the aperture. I think this is correct. I think when things are going well, I’m actually using the muscles surrounding the lips and not engaging the lip muscles themselves–I’m keeping those loose/relaxed to interact with the air column and vibrate as fast as possible (higher pitch). So I think it’s kind of a tug-of-war motion with the muscles where I’m reducing the aperture while I’m pushing back to keep the aperture from collapsing.

This is precisely the approach James Morrison discusses in this video:
https://youtu.be/ujrTDbnvDpU