The esoteric theory states that there is a substance in the air, called Absolute Energy, which is responsible for all activity, vitality, and life. This substance is called "Prana" and is all pervading.
Yoga breathing is called "pranayama". Prana, the life-force, is contained in the air we breathe and all living things, including the food we eat. It is a spiritual energy that departs the body when we die. When you learn about yoga breathing, you learn how to stimulate this life-force. Because Prana is everywhere, it's for this reason that focused yoga breathing connects the body, spirit, and mind as one.
When we breathe, we are constantly inhaling air that is charged with Prana. With proper yoga breathing we are able to take in even more, which is stored in the brain and nervous system. Yoga masters know that they can call on this life-source whenever they need it. Not only can they use Prana to strengthen their bodies, but also the mental faculties may reach an advanced level so that psychic powers are attained. Such people are often "magnetic healers" as they have the ability to impart vitality and strength to those they come in contact with.
Just as the oxygen in the air is used by the respiratory and circulatory system to sustain life, the nervous system also appropriates the Prana in the air and uses it to function. As oxygenated blood is carries throughout our bodies, Prana is delivered to the nervous system. And just as oxygen in the blood is used up by our internal body systems, Prana is depleted when we think, will, act, etc. Therefore, constant replenishing is crucial to our mental and physical well being. Every thought or motion uses up "nerve force" which is actually Prana. In order to lift your arm, the brain must first send a signal to the nerve, to make your arm muscles contract and move. When this happens, Prana is used up.
An important reason to learn about yoga breathing
The importance of Prana can't be emphasized too much. That's why taking the initiative to learn about yoga breathing is such an incredibly wise decision. Just imagine how you'll feel when every cell, every muscle, every organ in your body is functioning at peak capacity. Proper yoga breathing can do that for you.
Not only that, you're mental alertness and clarity will be much more focused and forceful. A controversial study suggests that we only use 10% of our brain. Imagine the possibilities when yoga breathing is used to tap into mental talents or abilities we may not even know we have.