Deep breathing has been shown to lower blood pressure, improve long standing patterns of poor digestion, decease anxiety, improve sleep and energy cycles, and allow people to wean off addictive anti-anxiety drugs.
To deep breathe, inhale deeply through the nose, mouth, or both (preferably the nose, so the air is filtered and warmed), dropping the diaphragm to fill the lower section of the lungs with air. Contract the abdominal muscles to create an antagonistic pressure called centripetal pressure. The lower abdomen should always be kept slightly tensed in a position of equilibrium between the centrifugal force of the breathing and centripetal force of the abdominal muscles. Near the end of the inhaling cycle, expand the chest to fill the upper section of the lungs. Inhale through nose and exhale through mouth. Keep tongue curled up so it touches roof of mouth. This keeps it away from teeth and lets saliva circulate so the mouth stays moist.
In normal breathing, the "topping off" is not used. You use just diaphragm and lower abdomen. This abdominal breathing pushes internal organs downward, which lowers the center of mass and increases stability. In chest breathing, the chest is inflated and the shoulders are lifted, which raises the center of mass and decreases stability.
It is possible to over breathe. When we are anxious or stressed, people advise us to "relax and take deep-breaths." However, deep breathing in a relaxed state causes dizziness and sometimes fainting. The cause of the O2 deficiency is not due to the lack of O2, but by the lack of CO2. Over breathing causes and O2 deficiency, so if we breathe too much, we have less O2 in our body. Without CO2, oxygen remains bound to hemoglobin, unreleased, and incapable of being used by tissues. As a result, there is an O2 deficiency in tissues, such as the brain, kidneys, and heart, as well as a significant increase in blood pressure. Ever notice how someone “holding his/her breath” becomes increasingly hyperactive. Over time, the level of CO2 increases dramatically causing the rapid consumption of O2. This hyperactivity may continue until unconsciousness.