The fourth type of pranayama transcends the external and internal pranayamas
Verse 2.51
बाह्याअभ्यन्तर विषयाक्षेपी चतुर्थः ॥५१॥
bāhya-ābhyantara viṣaya-akṣepī caturthaḥ ॥51॥
The fourth type of pranayama transcends the external and internal pranayamas, and appears effortless and non-deliberate.
Verse 2.52
ततः क्षीयते प्रकाशाअवरणम् ॥५२॥
tataḥ kṣīyate prakāśa-āvaraṇam ॥52॥
Pratayama removes the veil covering the light of knowledge and heralds the
dawn of wisdom
The fourth type of pranamaya goes beyond the regulation or modulation of breath flow and retention, transcending the methodology given in the previous sutra.
It is a state similar to kevala kumbhaka, which is mentioned in the hathayoga texts and in the yoga upanishads.
When the movement of the breath functions without one's volition or effort, the fourth stage of pranayama has been reached.
The movements of the mind cease. The flows of vital energy, intelligence and consciousness come to a standstill except for subliminal impressions.
This is like virama pratyaya, as explained in 1.18. A state of pause is experienced, in both the breath and the mind. From this springs forth a new awakening and the light of intelligence vigorously penetrates the sadhaka's innermost being.
Since this fourth stage contains no restrictions, it transcends the range of movements described in the pranamaya of 2 .50 verse. It is therefore a 'seedless' (nirbija) pranamaya.
Its practice destroys illusion, consisting of ignorance, desire and delusion which obscure the intelligence; and allows the inner light of wisdom to shine.
As the breeze disperses the clouds that cover the sun, pranayama wafts away the clouds that hide the light of the intelligence.
In the Yoga Chudamani Upanishad, it is said that there is no discipline higher than pranayama It is called an exalted knowledge (mahavidya) , a royal road
to well-being, freedom and bliss.
PATANJALI YOGA SUTRAS