Belly breathing or diaphragmatic breathing

Diaphragmatic breathing, abdominal breathing, belly breathing or deep breathing is breathing that is done by contracting the diaphragm, a muscle located horizontally between the chest cavity and stomach cavity. Air enters the lungs and the belly expands during this type of breathing.
This deep breathing is marked by expansion of the abdomen rather than the chest when breathing. It is considered by some to be a healthier way to breathe, and is considered by some a useful form of complementary and alternative treatment.

Diaphragmatic Breathing is also known scientifically as Eupnea, which is a natural and relaxed form of breathing in all mammals. Eupnia occurs in mammals whenever they are in a state of relaxation, ie when there is no clear and present danger in their environment.

How to exercise

Sit comfortably in a chair (or on a bed with your legs crossed beneath you). Place one hand on the stomach (the diaphragm or upper part of the belly where the chest ends) while simultaneously placing your other hand on the chest. Now breathe in deeply until you feel your stomach expand. Ideally, your chest should barely move. Alternatively, you can lie down on the bed and place something light (like a small pillow or a soft toy) on the stomach. The toy/pillow should rise up and down with each movement of your breath. This deep breathing exercise is very effective to fall asleep fast.

Explanation

According to the National Center for Complementary and Integrative Health, "12.7 percent of American adults [have] used deep-breathing exercises... for health purposes," which it describes as follows, "Deep breathing involves slow and deep inhalation through the nose, usually to a count of 10, followed by slow and complete exhalation for a similar count. The process may be repeated 5 to 10 times, several times a day."
According to the University of Texas Counseling and Mental Health Center, "Diaphragmatic breathing allows one to take normal breaths while maximizing the amount of oxygen that goes into the bloodstream. It is a way of interrupting the 'Fight or Flight' response and triggering the body's normal relaxation response."  They provide a video demonstration.

In complementary and alternative medicine

Some practitioners of CAM (complementary and alternative medicine) believe that particular kinds of breathing they identify as diaphragm breathing can be used to bring about health benefits.
Deep breathing exercises are sometimes used as a form of relaxation, that, when practiced regularly, may lead to the relief or prevention of symptoms commonly associated with stress, which may include high blood pressure, headaches, stomach conditions, depression, anxiety, and others.
Due to the lung expansion being lower (inferior) on the body as opposed to higher up (superior), it is referred to as 'deep' and the higher lung expansion of rib cage breathing is referred to as 'shallow'. The actual volume of air taken into the lungs with either means varies.

Relation to yoga and meditation

Hatha Yoga, tai chi and meditation traditions draw a clear distinction between diaphragmatic breathing and abdominal breathing or belly breathing. The more specific technique of diaphragmatic breathing is said to be more beneficial.

Benefits of diaphragmatic breathing

The use of diaphragmatic breathing is commonly practiced, especially in those patients with cardiopulmonary disease, to improve a variety of factors such as pulmonary function, cardiorespiratory fitness, posture, respiratory muscle length, and respiratory muscle strength. Specifically, diaphragmatic breathing exercise is essential to asthmatics since breathing in these patients is of the thoracic type in association with decreased chest expansion and chest deformity as a result of a shortened diaphragm, intercostals and accessory muscles from prolonged spasm causing stenosis of the major airways leading to an abnormal respiratory pattern.

Relation to singing

Proper breathing, from the diaphragm, is also essential to the best singing.

Source: https://en.wikipedia.org/wiki/Diaphragmatic_breathing