Practitioners of martial arts, especially karate, need to absorb and process the Ki that they are breathing in order to generate the power and force for the techniques they practice. They also need to be able to retain the Ki within the body until the moment it is needed. Basically, when we inhale we are bringing fresh oxygen and Ki into our body. When we exhale through the mouth we are expelling carbon dioxide which contains all the toxins and poisons that have built up within the lungs.
We are also expelling Ki from the body. But if we are continuously expelling the Ki we never give it a chance build up into the rich source of energy needed to complete our techniques to their maximum effectiveness. By exhaling through the mouth the Ki energy is simply dissipated back into the world. Breathing out through the nose, however, completes a closed circuit. By exhaling through the nose, the Ki energy, instead of being expelled with the carbon dioxide, is transferred to the dan tien or hara, located about three finger widths below the umbilicus. With each breath in, more Ki enters the body and circles down to the dan tien growing stronger and stronger. During this breathing process, the tongue is up, touching the top palate of the mouth just behind the front teeth and the air is expelled from the nose with a slightly audible hiss. There is also a feeling of the abdominal walls contracting down with the exhalation.
Once sufficient Ki has been generated this way the practitioner is able to expel the Ki with tremendous force. This is known as the Kiai where the breath is expelled through the mouth. This is the reason that there are usually only two techniques within each Kata where we Kiai. An important factor of Kata training is that it teaches us to build up sufficient Ki energy and then expel it in one strong technique. If we try to Kiai with every technique, we quickly become fatigued since we are expelling Ki with every breath.
Remember that Ki is a subtle, invisible force that requires much patience and long years of practice to understand. The ability to relax and breathe effectively will benefit your training in building Ki. When your mind and body are working together in a relaxed manner and you are breathing properly, a tremendous amount of energy is able to flow through your being. The key is not to force it, just slow down, relax and breathe through the nose.”
(Read more in Generating Ki Through Breathing)