One verse

Srimad Bhagavatam 3.28.9

3.28.9

प्राणस्य शोधयेन्मार्गं पूरकुम्भकरेचकै: । प्रतिकूलेन वा चित्तं यथा स्थिरमचञ्चलम् ॥ ९ ॥

pranasya sodhayen margam pura-kumbhaka-recakaih pratikulena va cittam yatha sthiram acancalam

pranasya
of vital air
sodhayet
one should clear
margam
the passage
pura-kumbhaka-recakaih
by inhaling, retaining and exhaling
pratikulena
by reversing
va
or
cittam
the mind
yatha
so that
sthiram
steady
acancalam
free from disturbances.

The yogi 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.