The movement is somewhat of a subtle thing and should only be as much as necessary, determined through experimentation. Less is more.

The corners of the mouth don’t seem to move much to me, there is more of a sensation of engagement/tension in the corners while the middle part of the lips are free to vibrate.

To me, it feels more as if the vibration is happening more and more inside the aperture as I go higher. I was kind of working on a bullseye graphic of concentric circles with colored patterns in each gap between the circles but can’t figure out how to paste it in here. Here’s a link to a bullseye diagram:

https://images.app.goo.gl/r1zeN5tQuxGzCBLUA

Pedal tones might be on the outside (white) area, the black rim area may be low register, the blue rim may be middle register (on the staff), red area might be upper register with High C and beyond in the innermost Yellow band.

To do this type of graphic right, though, it would probably need to be more of a 3 dimensional graphic though and the yellow band would be more inside the aperture (i.e. closer to the teeth than the mouthpiece rim). This is MY experience and may vary with others.

Early on, I overdid the “aperture corner” enagement thing and took it way too literally and thought of the aperture corners as the corners of the mouth, resulting in too much of a pucker and too much mouthpiece protrusion–this made it difficult for me to articulate, manage consistency, control (i.e. dynamics), etc. I could obtain the pitch(es), but couldn’t use them much.

As I worked on making harmonic slurs more efficient and focusing in on each pitch more and more, and working on fast articulation throughout my newfound range, I learned to do less which wound up being less and less of a “pucker” and more and more of a subtle engagement of the aperture/corners.

This is my $.02, FWIW. Hopefully others will chime in on this as well. But hopefully something in there is helpful to you.