Diaphragmatic breathing (sometimes called Belly Breathing) is a technique for increasing  the oxygen the body needs during times of stress.  During times of stress, some individuals have shallow breathing which inflates the upper part of the lungs using the accessory muscles.  Shallow breathing disrupts the balance of oxygen and carbon dioxide and can increase anxiety.  

Watch a healthy sleeping baby breathe or watch your dog or cat breathe.  They have it right - they are using diaphragmatic breathing.  Most healthy individuals are using diaphragmatic breathing while sleeping. Singers, athletes, and musicians playing wind instruments all use diaphragmatic breathing.

Diaphragmatic breathing is also helpful for relaxation, reducing the perception of pain, and inducing sleep at bedtime.   It is particularly helpful for individuals with chronic respiratory problems such as asthma and obstructive lung disease.  

In addition to increasing the oxygen your body is receiving, focusing attention on breathing helps to put other thoughts and worries aside for a brief period.

It requires no equipment and can be used  anywhere.   It can be used before, during, and after stressful events. It can also be taught to children. 

Like any relaxation / stress management technique, it must be practiced in non-stressful situations, so that it becomes well-learned and can be then used in stressful situations.