Patanjali Yoga Sutra — बाह्याभ्यन्तरविषयाक्षेपी चतुर्थः ॥2.51॥

Bāhyābhyantaraviṣayākṣepi caturthaḥ ॥

In the fourth method of prāṇāyāma sadhaka experiences kevala kuṃbhaka, which is unforced (spontaneous) stoppage of breath resulting in unprompted cessation of thoughts.

Indian traditional texts place the Prāṇa at a very high pedestal. In Praśnopaniṣad there is a prayer on Prāṇaśakti:

प्राणस्येदं वशे सर्वं त्रिदिवे यत् प्रतिष्ठितम्। मातेव पुत्रान् रक्षस्व श्रीश्च प्रज्ञां च विधेहि न इति॥२.१३॥

Prāṇa controls everything existing. All that exists in heaven is also controlled by the Prāṇa. O Prāṇa, please protect us as a mother her children. Please give good fortune and wisdom.

There is an interesting story on Prāṇa in Cāndogya Upaniṣad (5.1.6–14):

Once sense organs began quarreling among themselves to claim supremacy over each other. Each one of them said, ‘I am the supreme, I am the supreme’. The dispute was brought to their father Prajāpati. He gave his verdict, ‘the best among you is the one on whose departure, the body gets decomposed and becomes untouchable’.

In order to prove its supremacy, the first speech left the body. After staying away from the body for one year, speech returned to the body and asked, ‘in my absence, how did you sustain?’ The rest of the organs said, ‘just as mute people live without speaking. We survived by breathing, by seeing, by hearing, by thinking’.

Next, the eyes left the body for one whole year. Having come back to the body, asked how others were sustaining in the eyes’ absence. Organs said, ‘just like blind persons. We survived by breathing, by speaking, by hearing, by thinking’.

Next, the organ of hearing left the body for one year, then returned and asked, how other organs were sustaining in the absence of ears. Organs said, ‘just like deaf persons. But we sustained by breathing, by speaking, by seeing, and by thinking.

Next, the mind left the body. After one year returned and asked how other organs were sustaining in the absence of mind. Organs answered, ‘just as children do without thinking for themselves. But we were able to survive by breathing, by speaking, by seeing, by hearing’.

Now the prāṇa, decided to leave the body. But just as a strong horse is able to uproot the pegs to which his legs are tied, the prāṇa started carrying all other organs with him. Thus, having accepted their defeat and realizing that they are dependent on prāṇa for their survival, other sense organs prayed the Prāṇa with all humbleness, not to leave the body.

Organ of speech said, ‘though I am of very high importance, it is because of you I am important’. Eyes said, ‘though I can support the body, it is because of you I am supporting’. The ear said, ‘though I am affluent, it is because of you I am affluent’. The mind said, ‘though I have the quality of sheltering many, it is because of you I am sheltering’.

Coming back to this aphorism, in kevala kumbhaka, sādhaka experiences effortless cessation of breath owing to his prolonged sādhana. Interestingly, even the mind goes to a deep blissful silence so long as there is kevala kumbhaka. Vyāsa Maharṣi says in his bhāṣya on Pātañjala Yogadarśana, in this fourth aspect of prāṇāyāma, though being affected by the viṣayas, having stopped the twin movements of prāṇa, the sādhaka transcends the influences of viṣayas.

However, post kapālabhāti, for fraction of a minute, even a beginner can experience kevala kumbhaka and all associated changes. Physiology edifies us that, Kapālabhāti practice amazingly builds up prāṇa level in the cellular body and for the next few seconds, cells of the body, having attained saturation, do not need any more prāṇa supply. Hence, breathing stops for a while. But the cells will be continuously using prāṇa thereby prāṇa level gets depleted and breathing slowly starts after a while.