It is important to note the contraction of the diaphragm (the primary mover of breath) expands the volume of the torso and thereby pulls air into the lungs. The diaphragm is a dome shaped structure with a central tendon and muscles that attach to the inner walls of the ribcage and the spine. Think of a trampoline with it's bouncy middle and springs attached all the way around it. The springs are the muscles contracting (inhale) or relaxing (exhale). In the picture above the central tendon is white and the muscles are red.
Here is an excellent video which is only 6 minutes long. It is done by Donna Bervinchak, Feldenkrais & Child'Space practitioner, in Lancaster, PA. You will feel an effortless, multi-directional breath after you do this lesson. It is very effective for anyone with neck, rib, shoulder and back tension, as well as those with abdominal/ stomach tension. It is useful for blood circulation since the aorta travels through the diaphragm.