Its fibers attach to the inner lower six ribs, the inner xiphoid process and the first three lumbar vertebrae and then come together at the central tendon. When this muscle contracts and pulls down on the central tendon, it creates inspiration. The central tendon has connections to the connective tissue that surrounds the lungs; thus, when the muscle contracts, a vacuum develops in the upper thoracic cavity and pulls air into the lungs. When you exhale, the diaphragm relaxes and the lungs deflate. To simplify this, the diaphragm is a major player when it comes to breathing. This is all lovely but just because the muscle is there does not mean that it is functioning optimally.
During my massage therapy studies up to this point, one very common complaint people seem to have is pain in their posterior shoulder girdle, neck and various areas of the mid to upper spine. One common issue I have seen with these people is poor breathing patterns. If diaphragmatic breathing is occurring, the following test will confirm it. Place one hand on your stomach and one on your chest. If your stomach pushes out first and strongly, the diaphragm is doing its job. If the chest rises first, it is not. When it is not working optimally, other muscles that have their own primary important jobs try to assist with breathing and can either strain themselves or cause problems in other muscles around them from compensation. For example, muscles in the upper back and neck such as the scalenes and upper trapezius can strain when more stress is placed upon them without full diaphragm function. Erectors and deeper spinal muscles can also strain when poor diaphragm function causes poor rib movement.
When it comes to performance, proper diaphragmatic breathing also becomes important. In order to perform at the highest level possible, an abdominal brace must be maintained during exertions and movements in order to prevent energy leaks. If the diaphragm is not working optimally, the abs will brace and unbrace along with the breathing rhythm. The side bridge can be used as a good example here. If somebody is in side bridge position and you feel their obliques or rectus abdominis, these muscles should maintain a decent brace (they should be "hard" and pushed out) throughout the breathing cycle. The diaphragm should take care of the breathing efficiently enough so that these muscles can maintain the brace in order to facilitate better performance of the bridge. This is the same thing that needs to happen with many other sporting activities.
Diaphragmatic breathing can be practiced multiple times a day. Place one hand on your stomach and one on your chest and practice breathing into the hand on your stomach for multiple breaths throughout the day. This can help with pain management and performance more than you'd believe. You can also train each side to function optimally by placing a hand on your left and right lower ribs and practice pushing each side into each hand alternating. This is a great exercise as there are many people who have one side that works better. Patrick Ward and Bill Hartman both have some great info on this stuff and could tell you about the subject better than me so if this interests you be sure to check out their stuff. Both of their sites can be found in my links page.