Thanks for this precision. I think that i was doing that. Thinking to change pith by firming or relaxing the corners in a inward or outward movement. Finally i think that it is a simple movement of the corners moving toward the mouthpiece (like saying toooo) and letting the center of the lips free to vibrate. I have the feeling that this movement involved the muscles of the jaw and also the chin that are cooperating synergistically.
What do you thing about that? Kind regards. Francis.