Well worded question, Miguel.

That is the “$1 million” question. It cant really be answered any more clearly than Greg outlines in WindWorks, it just takes time for each of us to realize it our own way–as you noted, its impossible to really know what our lips are doing in the mouthpiece precisely.

But there are clues that we can pay attention to and observe during our practice, experimentation, etc.

The notes speaking arent necessarily an indication that theres no clamping. Its a spectrum, really, that we’re dealing with–on one extreme, there is no clamping and a resonant, easy/efficient sound and at the other end, there’s a strained, tight/thin, inefficient sound that is nearly cutoff.

Pulling the mouthpiece away from the horn as we’re playing is a good clue–are we getting a sympathetic buzz when releasing passive air into the horn, or is it an active buzzing on the mouthpiece.

Its a spectrum we are on and we constantly want to improve our efficiency and effectiveness.

And we have good days, great days and bad days and awful days.

Those 3 points above are key, youre on the right track. Focus on releasing passive air and engaging the aperture corners just enough for the next pitch to speak.

We need to gradually tighten the aperture inward towards the air column / hole in the center of the mouthpiece while keeping the top and bottom lips as relaxed as possible so they vibrate as freely (fast) as possible, but only as much as necessary. By focusing on the aperture corners, engaging them inward horizontally from the sides rather than clamping down top-to-bottom like a clam, we play more efficiently and get a better sound easier.

My $.02, FWIW, based on my experience / interpretation, etc.

Hope that helps, best of luck!