Breath is the force of life. In buddhism they believe that the breath is the connection between the mind and body. It is the vital force of life. When we are born we breathe from the diaphragm. If you watch a a baby breathe, you will notice his or hers stomach move up and down. With time most of us learn how to breathe with the upper chest and forget how to breathe correctly. If you watch UFC fighters and pay attention to how they will breathe, you will notice how their stomachs go up and down when they breathe. Elite athletes breathe from the diaphragm and it is a necessity to perform at the high levels of competition. Practices like yoga, thai chi, systema, and chi gong know the importance of breathing and emphasize this practice.
Steve Maxwell in video talks about how we should breathe and how incorrect breathing can shed years off our life span. Check it out.