This is a very important and an interesting question in Yoga practice. Basically, in Yoga practice a very important tool is Pranayama because we are working directly there with Prana, our energy system. The way we work with our energy system is through regulation of breath, conscious regulation of breath. We have to extend the inhale, we have to extend the exhale etc.
A long inhale and a long exhalation is representative of a very calm and steady mind. When you are excited or when you are distracted or when you are disturbed the breathing will become shallow and short. Whereas, when we are focused and when we feel relaxed you will see that the breath will be much longer; much more quieter. You can see this in babies. When babies are in the comfortable arms of their mother they are relaxed and the breathing becomes very very smooth, very very long. compared to them of course. Whereas when a stranger carry the baby, when he comes and takes the baby, the baby gets disturbed and you can see that the breath starts to get short. So breath extention is not for extending the breath like the championship as to who has the longest breath. Breath extension has consequences into our mind structure so that the mind becomes more relaxed and therefore in turn then the has an impact in our physiological and emotional structures as well because we are a holistic entity.
But breath cannot be continuously extended. I can’t just extend my inhale and exhale as I want. So, at some point we need to educate our respiratory system to become able to extend the breath. One of the tools that allow us to extend the breath is retention after exhale and retention after inhale. The retention after exhale vitalizes the exhalation and the retention after inhale vitalizes the inhalation. At some point when we are not able to extend the breath further, say after inhale or after exhale, we use the retention after exhale and the retention after inhale to vitalise the breath for the current moment and over a period of time you will see that by extension of the retentions after exhalation, the retention of the breath will become possible because the muscular capacity of the respiratory system will get trained in this manner. This is one of the main reasons why retention after inhalation and the retention after exhalation were suggested.
There are many other reasons as well. But the scope of this video prevents us from going into such kind of detailed explanation.
Dr.Kausthub Desikachar, Viniyoga® Teacher