One of the most detailed and accurate explanations of diaphragm and breath control I have read and I highly recommend this way of teaching
If you use "the lower stomach system initially to inhale" your "diaphragm is pulled down on the inhaling effort by the (weight of the) lower stomach muscles". This approach to inhalation is often coupled with a breath control exhalation method of "pushing downward and outward on the abdominal viscera, much as in difficult defecation" (Miller).
I do NOT suggest any part of the above as a correct method of breath control.
What I do propose is that you inhale from a position of correct, erect posture with the sternum in the elevated position that it assumes if you raise your arms over your head as you inhale and, holding that breath with the inspiration muscle system (not the closed vocal folds), lower the arms to your sides and slowly exhale without lowering the now elevated sternum. This should give you the correct posture for the singing breath.
Each subsequent inhale is accomplished without additional raising of the rib cage but with a sense of expansion or fullness in the epigastric area (that area between the bottom of the sternum and the navel and extending outward to the ribs on each side). If you are long-waisted you will have less sense of this epigastric expansion because there is more vertical room in the long-waisted abdominal area for the viscera and less outward expansion is the result.
Each inhale will also be accompanied by a sense of expansion of the lower ribs. This rib expansion is caused by the contraction of the external intercostal muscles and should feel a quite natural part of a substantial inhale.
All of the above gives a common felt sense of "suspension" at the height of your inhalation when you are breathing deeply.
When you begin to sing this sense of "suspension" should be maintained as long as is comfortable, with the sternum still elevated, the epigastric still comfortably full, the lower ribs still expanded. It is this position that prevents the diaphragm from collapsing upward too quickly. The abdominal muscles should be relaxed and you will find the necessary exhalation will occur without your having to be overly concerned about the action of the muscles in the abdominal area. As you arrive at about the last 1/3 of your exhaling breath you will, naturally, feel your epigastric move slightly inward but you should attempt to keep the lower ribs in as outward a position as you can. Yes, this is a learned response but it is the only way to deter the early ascent of the diaphragm.
To be more technical, the weight of the abdominal viscera (your lower abdomen area) pulls downward against the upward pull on the diaphragm created by the negative intrapleural pressure in the thorax as air is exhaled. The diaphragm remains relaxed and is acted upon rather than being active. The amount of the abdominally caused downward hydrostatic pressure is dependent on the perpendicularly size of the abdominal cavity. This size is controlled by the exterior intercostals. The wider the rib opening and the longer this expansion can be maintained, the greater the downward hydrostatic pressure and the greater the pull against the elevation of the diaphragm. Consequently it is this rib expansion that becomes the most conscious effect for the singer. And we do get stronger and more adept at controlling and maintaining this lower rib action. This lower rib expansion and the epigastrice fullnes which, in turn creates the feeling of inspiration suspension, is "appoggio".
Can all the above create tension? Of course! Should it? No! At least not the tension of major muscle fighting major muscle.
Lloyd W. Hanson, DMA
Professor of Voice, Pedagogy
School of Performing Arts
Northern Arizona University
Flagstaff, AZ 86011