The mantra. I may be misusing the word but I like to think of it as short hand, rather than get bogged down in finer and finer semantic musings. It's not that I cannot handle technical details. My job at work has some very fine technical details to it, every day. But, for some reason, with singing, the simpler I make it, the more flexible it is, for me.
Motion, when necessary, in the abs.
Note in the head.
Nothing in the throat, ever, amen.
And now, for finer definitions. Again, language is the problem because of how others view the words I say from their own perspective, desires, aesthetic considerations.
When I say abs, I don't mean specifically the abdominus rectus, or even abdomina oblique. If a lateral image of breathing helps, so be it, as long as the action is low in the thoracic area of the body. If nothing else, it is to avoid chest-heaving. Someone else once pointed out that breathing into the back or the gut (I think it was Daniel Formica, a genius if there ever was one) may depend on how your hips rotate back or forward. Someone who's hips roll back may need to feel breathing in the back, as trying to feel it in the belly brings too much tension. And the converse, the person who's hips roll forward gets more benefit from a buddha belly than trying to breath into the sacro region.
Here is the important part of the first phrase. "When necessary." Sometimes, more movement is necessary, sometimes less. And some are going to want exact measurements, like a list of torque specifications on switchgear made by Square D. And I can't help you with exact movements in the ab region. Just to know that is where the movement, if any is necessary must be. For that leads me to another amateur point of mine. The noble chest.
There is some instruction to keep a noble chest posture. I think that is the cart before the horse. A good note often accompanies a noble or non-collapsed chest or an absence of chest heaving. And that is my point. If your motion is in the abs or laterals or whatever, then it is not in the chest, which allows the chest to remain "expanded" compared to collapsed. The noble chest is a side effect of proper breath management, not the cause of it. Though some valuable exercises help isolate that feeling, such as singing while lying down and allowing the feeling of movement in the belly and sides while the chest remains still. To assume that the note comes from purposefully holding the chest "noble" is akin, in my mind, to saying that Nike shows make you trim and fit. When the reality is, a number of professional athletes, who are trim and fit from their athletic endeavors, also happen to wear Nike shoes. The fitness and the shoes are coincidental, not cause and effect.
Is it a massive crush of air from below? No. It is a controlled exhalation and not a holding of the breath as if one were swimming under water. But does mean slowing down or metering the exhalation of air. And there can still be some air pressure, which is more important than air speed. Most people speak using residual air pressure from collapsing lungs. Singing requires subtly more control than that. And that has to do with controlling what you do with the muscles below the rib cage, not your throat or chest.