Natural and Reverse Breathing.
To breathe in naturally the muscles of the diaphram and the thoracic girdle contract and to breathe out naturally these muscles relax. This is the natural, physical basis of both Reverse Breathing and Natural Breathing. In unconscious breathing the rest of the body remains relaxed throughout the process so that the abdomen will expand on the in-breath and fall back on the outbreath. This is then termed Natural Breathing and is used whenever the body is inactive as in deep meditation. To this process is sometimes added the taking of the mind to the lower Dantien when breathing in and the rising and expanding of the mind on breathing out.
When the body is active however it is natural to accompany the in-breath and contracting diaphram and thoracic girdle muscles with a contraction of the active muscles of the body as well as a slight contraction of the abdominal and pelvic floor muscles resulting in a slight drawing in of the abdomen and pelvic floor. Combining this with drawing the mind and the energy in and up through the body to the top of the head during the in-breath is termed Reverse Breathing.
Sometimes taught for the sake of power, but not to be recommended from the point of view of bodily health or energy circulation, is the unnatural forcing out of the breath with a sharp contraction (as at this time the body should naturally be relaxing) typically accompanied by a loud shout or rasping noise from the throat. Occasionally to this is added the drawing down of the mind to the abdomen on the in-breath and the expansion up through the body to the hands during the contracted out-breath.
In both Natural and Reverse breathing the out-breath naturally triggers an instinctive relaxation response in all muscles throughout the body and this is deliberately used in the initial stages of both emptying the body of strength and de-activating the superficial mind while in the process of deepening the consciousness.