Śrīmad-Bhāgavatam · Text 3.28.9
Who is speaking and story frame
प्राणस्य शोधयेन्मार्गं पूरकुम्भकरेचकै: ।
प्रतिकूलेन वा चित्तं यथा स्थिरमचञ्चलम् ॥ ९ ॥
prāṇasya śodhayen mārgaṁ
pūra-kumbhaka-recakaiḥ
pratikūlena vā cittaṁ
yathā sthiram acañcalam
The yogī should clear the passage of vital air by breathing in the following manner: first he should inhale very deeply, then hold the breath in, and finally exhale. Or, reversing the process, the yogi can first exhale, then hold the breath outside, and finally inhale. This is done so that the mind may become steady and free from external disturbances.
Read Śrīla Prabhupāda's purport on Vedabase.io