By the time you have gotten to the point of practicing Pranayama regularly, you take your practice seriously. Pranayama is the end of the line of the physical practice. Like approaching the end of the high dive board, this is where the practitioner takes the “plunge” from the known physical practice, to unknown internal practice. In short, Pranayama is the most spiritual physical practice one can do.
Sutra 1.34 says: pracchardana-vidhāraṇābhyām vā prāṇasya, or the exhale and retention (rechaka and bahya kumbhaka) are a means to cease citta vritti (mind chatter). As you may remember from Sutra I.2 stopping the mind chatter the main aim of Yoga practice. B.K.S. Iyengar in Light on Pranayama (1999 ed.) takes this Sutra’s concept a step further:
Exhalation is the process by which the energy of the body gradually unites with that of the mind, merges into to soul of the sadhaka and dissolves into cosmic energy. It is the path of return from the peripheries of the body towards the source of consciousness know as the path of renunciation (nivritti marga). (Page 100) and Bahya Kumbhaka (retention on the exhale) is the state in which the yogi surrenders his very self, in the form of his breath, to the Lord and merges with the Universal Breath. It is the noblest form of surrender, as the yogi’s identity is totally merged with the Lord. (Page 106).
Here, Iyengar may be referring to the “fourth” type of pranayama found in Sutra II.51. This “fourth” type of pranayama “surpasses the limits of the internal and external.” Then uncovers the thin veil between ignorance and illumination. Then the practitioner is qualified for Dharana (paraphrasing Sutras II.52-53).
In short, these Sutras are telling us that Pranayama is a destroyer of Karma which is a fundamental “goal” of the serious yoga practitioner. And like Pranayama cannot be commenced until Asana is mastered, the Sutras are suggesting that one must be proficient in Pranayama before intensive concentration (Dharana) can be undertaken.
I am very far from this level of progression in my personal practice. But the reason I am posting this is that it seems most Yoga we see and read about are just addressing Asana for Asana’s sake. In other words, “30 days to master scorpion pose” which we see ubiquitously on the Yoga news feeds of Facebook and WordPress. Perhaps this post will show that there is a progression along the eight limbs, and that each rung, like a peak of a mountain, must be ascended with much preparation and awareness. That in itself is an extremely spiritual undertaking.