The bhastrika pranayama directly affects the nervous and circulatory systems. The intracranial pressure abruptly increases through each inhalation which, in turn, magnetises the cerebrospinal fluid.
The subtle body is inherently closed for a significant reason. Once it is made open, the subconscious awakens which in turn emerges our karmas. Hatha Yoga is the most powerful in awakening the kundalini. It is also the most dangerous.
Once the nada is heard, slowly pierce through it until a softer sound appears. Focus on this new sound. Then go past the new sound and another one shall emerge. The nada is inside the mind only and the sound is to be heard in the mind only. We cannot create these sounds, they are already there. We have to attune our minds for the nada to manifest.
Finally, we must learn how to breathe through the Union of the Soma and Kundalini. There are actually two places where we could do this practice. One is inside the heart and the other one occurs through the third eye. Feel the inspiring of the breath open the space of the heart and allow it to melt and relax through each exhalation. When we breathe through the heart, the mind and breath are calm and balanced.
The mind emerges the energy of the soul. This spiritual force is the most healing, for ourselves and others.
In a functional perspective, the goal of pranayama is to refine and make the natural breath healthier, balanced and more efficient. Pranayama purifies the Nadis so that the prana (life force) can freely flow through them, nourishing and recharging our vital organs. Further still, deep and meditative pranayama stills and calms the mind in preparation for the state of Samyama (concentration, meditation and absorption).