I am a retired science teacher. I experiment. If I try playing without engaging lip corners. I get another beginner sound I’ve heard before.
Try this: Do the aah-ooh and then try to whistle. You may not be able to whistle at all, but if you are going to whistle, you will use some lip corner tension.
Try this: Form the aah-oooh and sick the eraser end of a pencil in your aperture. The eraser end should be in front of your teeth. Now try to get the pencil more perpendicular to your lip by using “lip corner” tension along with aperture corners (next to the pencil). Do not clamp your lips down trying to get this pencil up, this is the wrong way.
The whole idea behind the aah-oooh is to not have center lip tension or clamping the lips together and that is all that the aah-oooh is getting a person to avoid. When I used to play high by clamping the lips, I had to blow with a lot of pressure to get the air out of the lips and sometimes this would cause my cheeks to try to puff out and I had to push the horn into my face. It worked for that one high C I blasted out but then I was not going to play another or much of anything else unless it was in the staff or below the staff.