I just watched the following video in the WW Course and thought of you, this thread, etc.
Andante Fundamentals – Aperture Corner Development
I can’t link the video as it’s part of the course and not a free video, but I recommend you go back and watch that. I think Greg did a great job explaining how to get the corners in–your question.
Also, note that Greg points out the fact that the jaw does not need to move when the corners tighten. I think you mentioned your chin moving–that might be ok, or it could be a problem if you’re moving your jaw too much you might be closing your teeth too much behind the aperture and cutting the air off.
The tongue arch can be helpful, but Greg points out that the tongue arch does not change pitch–it helps the resonance of the sound. The tongue arch can be helpful, for sure, but the change of the shape of the aperture is what changes pitch. You can change pitch by tightening the aperture without arching your tongue. And the tongue arch, if done incorrectly can actually get in the way of the air if we’re overdoing it.
Lately, I have actually been doing some Schlossberg harmonic slur exercises in which I have been trying to play them as open as much as possible and have found myself using very little, if any, tongue arch–maintaining an Aaaahhhh or Oooohhh (not Eeeeeeeeee) sensation even up to High C. I really try to keep things as open as possible; the exercise is played very softly.
I wouldn’t get too caught up in what the tongue is doing necessarily, but it can be a problem. I usually just go with what I feel is natural. But I have noted that many great players (i.e. Herbert L. Clarke, Greg Spence, Jens Lindemann, Claude Gordon, etc.) use “anchor tonguing” or KMT tonguing in which they anchor their tongue behind their bottom teeth. While they do arch their tongue in the upper register, my understanding is that they’re somewhat doing that to ensure that the tongue doesn’t get in the way of the air–especially at the most important part, where it meets the aperture itself.
I haven’t progressed to the point where I think I need to worry too much about whether or not to do anchor tonguing / KMT; I played around a bit with it and couldn’t make sense of it. It might not be something I need to do; many players don’t.
But I haven’t mastered my newfound range yet to the point where I can play it with every articulation, dynamic, etc. So, it’s possible as I try to develop further that I will conclude that will be necessary.