The air we breathe in and out is sthula prana, the gross air. When this gross air enters the body, it becomes sukshma prana, the subtle air performing various types of activities inside the body.
Kumbhaka or the retention of the breath should be practised in a disciplined manner. There should not be any irregular or casual attempt to hold the breath because the air you are trying to control is powerful.
The level of the practice and achievement in it is seen in the body of the yogi and its appearance. There will be a change at each level of Hatha Yoga and its practice and a different grace in the body.
Health is most important and the practitioner should first give attention to having good health. All your activities depend on your health condition.