In diaphragmatic breathing the movement is largely that of the diaphragm moving down on the inhalation and up on exhalation. There is also specific involvement of the abdominal muscle wall, resulting in the lifting and expansion of the lower rib cage on inhalation, because the taut abdominal muscles confine the incompressible viscera as the diaphragm descends. It is the involvement of the muscles of the abdomen and the external intercostals (which flare the lower ribs on inhalation) that distinguishes the diaphragmatic breath from the abdominal breath.
The effect of the diaphragmatic breath is a stimulation of the sympathetic system (a general activation and warming of the body’s autonomic systems), and the mental effect is that of clarity and attentiveness. This is the appropriate breath for most yogasanas. (Chapter 15, pg 612)