Breathing has two main components: in and out.
In singing, we want the outflow of air to last as long as possible.
Resisting the outflow of air involves creating pressure in the lungs, which I call compression.
Expansion. Relax thorasis & engage external intercostals.
Contraction. Resist contraction of internal intercostals, delay contraction of thorasisis. Allow viscera (stomach area) to move in.
Compression, put simply, is squeeze. We want the least amount of squeeze necessary to accomplish our goal.
We resist the outflow of air at the point of the vocal folds, and we increase the amount of squeeze with our diaphragm.
The tricky part is that as we sing, we are letting air out - so we have to increase the pressure gradually, gently, yet inexorably.
Appoggio is, in part, that battle between the inhale and the exhale. Below is a very excellent description of appoggio.
Appoggio - from the Italian verb appoggiare, meaning 'to lean on', 'to be in contact with' or 'to support' - is a learned breathing technique that involves slowing down the ascent of the diaphragm for better breath management, resulting in the elongation of the breath cycle during singing. It involves a concerted action on diaphragmatic movement by the muscles of the thorax (chest) and the abdominal wall (the transverse abdominis, the internal oblique, the external oblique, and the rectus abdominis, although to a lesser extent), and involves gaining better control over the breathing mechanism through training the muscles, and enables the singer to pace the breath more efficiently.
It's important to note that appoggio, while requiring a great deal more control than is needed during normal activities and requiring some additional coordination and training, is an extension of the natural breath process, not a substitute for it. It is considered to be the main route for breath management within the international classical singing community.
During normal speaking, the rib cage collapses upon exhalation. Appoggio attempts to avoid this collapse by retaining the elevated inspiratory posture of the rib cage and the sternum - the long flat bone located in the centre of the thorax (chest), which connects to the rib bones via cartilage, forming the rib cage with them. The technique slows down the rising of the diaphragm, which aids in breath management. Learning to gain control over the muscles of the side abdominal wall offers true breath support.
Appoggio involves raising the sternum before inhalation, making all conscious breathing efforts with the internal and external obliques and transverse abdominis, then keeping the sternum up and not allowing the chest to recoil when replenishing the air supply. (This is sometimes called 'sideways inhalation'.) [snip]
One method of achieving this ideal elevated sternum posture involves raising both arms above the head - the sternum naturally rises when the arms are in this position - suspending, but not holding, the breath with the inspiration muscle system, then lowering the arms while exhaling slowing without lowering the sternum. Assuming the Garcia position, with the palms facing outward and placed one on top of the other on the sacrum - not the small of the back, which could cause the back to 'sway' and become overly arched - while vocalizing can also encourage an opening of chest wall with a raised sternum.
If applied appropriately, simulating the posture of breath retention can minimize subglottic pressure. At inspiration, subglottic pressure is at its lowest level and lung volume is at its highest. The lowering of the diaphragm and the widening of the lower ribs causes the dimensions of the thoracic cavity to increase in both length and width. Due to its incredible elasticity, the entire respiratory tree stretches downward with the descent of the diaphragm, allowing for greater lung capacity, as the lower lobes of the lungs are now able to be filled. (Pushing upward with the abdominal wall and forcing the diaphram upward compresses the lungs and decreases their capacity.) Staying in the position of initial inhalation gives the singer the sensation of 'singing on the gesture of inhalation' - we do not inhale as we sing, but we are merely making the gesture, or retaining the same posture - rather than on that of exhalation, which retards breath exit, and in turn retards the return to atmospheric pressure and minimizes mounting subglottic air pressure.
In singing, (as well as in the Italian language itself), the term 'appoggio' has both a passive component and an active connotation, and may vary with different technical approaches. For some singers, breath flow pressure becomes a self-sustaining system whereby the singer feels the breath pressure in the body as an influence of stability. The diaphragm remains relaxed and is acted upon rather than being active. Others, however, might say that they consciously push down against the pressure of the breath. In that way, they are actively trying to find something to 'lean on', to use as a support. (I find that many of my students initially experience the latter sensation, in which they are more aware of the sense of pressure in their abdominal, side and back muscles, and then gradually begin to find the technique easier, more natural and more beneficial over time as their muscles become stronger and better coordinated with what is happening in their larynxes.)
Inhalation should be accompanied by a sense of expansion or 'fullness' in the epigastric area, as well as a sense of expansion of the lower ribs. This rib expansion is caused by the contraction of the external intercostal muscles, and should be felt during any substantial inhales. In addition to expansion at the base of the ribs, it can be felt at the front and sides of the torso, between the tenth rib and the crest of the iliac (upper surface of the hipbone) and in the back at the eleventh and twelfth ribs. 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. Lateral abdominal expansion will eventually equal or even exceed the expansion of the front part of the abdomen when the appoggio system is developed and applied. Consequently, this rib expansion is the effect that is typically most noticeable to the singer.
At the height of inhalation, when the singer is breathing deeply and the lower torso is expanded laterally, dorsally and frontally, he or she will likely also feel a sense of 'suspension', in which it feels as though the voice is sitting or resting on something, or a feeling of 'buoyancy'. When he or she begins to sing, this same feeling should be maintained for as long as is comfortable, with the sternum still elevated, the epigastric area still comfortably 'full' and the lower ribs expanded. (It is this position that prevents the diaphragm from rising upward too quickly.) The abdominal muscles should be relaxed, and the singer will find the necessary exhalation will occur without the singer having to be overly concerned about the action of the muscles in the abdominal area. As he or she arrives at about the last third of the exhaling breath, the epigastric area will naturally move slightly inward, but he or she should attempt to keep the lower ribs in as outward a position as possible, without thrusting the muscles outward or downward. This is a learned response that will help retard the ascent of the diaphragm or avoid its early rise.
This lower rib expansion and the epigastric 'fullness' which, in turn, create the feeling of inspiration suspension, is [part of] "appoggio". [The other part is laryngeal structure]
At first, it may seem as though you are not getting enough air, especially if you are accustomed to hyperextending the support muscles then thrusting them upward and inward as you sing, but you will soon find that your air supply is indeed sufficient to complete your singing tasks, even long phrases, because the diaphragm is rising slowly and pacing the exit of the air to make it last for the duration of the tasks. You will get stronger and more adept at controlling and maintaining this lower rib action.
Acquiring the appoggio breathing technique gives the singer a longer, more reliable air supply (because the exiting air is pragmatically paced in order to meet the requirements of extended phrases, regardless of tessitura or dynamic level), greater stability of tone (because tone is affected enormously by the steadiness of a singer's breath stream), easier execution of large intervals, improved agility, including greater clarity, accuracy and speed while singing technically challenging passages, and better breath management when singing very softly or quietly. Appoggio ensures that there is neither excessive airflow, (because most of the exiting breath is turned into tone by the efficiently vibrating larynx), nor too much resistance by the vocal folds to the exiting air (la lotta vocale).