The diaphragm is a muscular partition between the thoracic and the abdominal cavities. Its location, in the front, is at the base of the sternum [breastbone] and in the back on the spine and at the base of the rib cage. During contraction, the diaphragm descends, the chest cavity is enlarged and air pressure is lowered. This is responsible for 75 percent of the normal volume increase of the lungs. It is a physical impossibility to use the diaphragm to raise intrathoracic pressure. For deeper breathing the ribs are elevated and expanded outwards, further expanding the chest by the external intercostal muscles. Small increments in volume can be obtained by further elevation of the ribs by muscles of the neck and back.
Many teachers use the phrase, “blow from the diaphragm,” and use the term “diaphragmatic support.” “The term ‘support’ raises questions in itself. Many people make the mistake of assuming that muscle contraction is what provides support. The blowing of the breath should be the support, not tension in the muscles of the body, but the movement of air that is required by the embouchure or reed.