Yoga Rahasya Chapter Two verse 30

yayā vṛtyā prāṇavāyoḥ dairghyaṃ dine dine|
vardhete ca viniḥ bādhaṃ vṛttiṃ tām-anusaṃcaret ‖

Encompass that which promotes
the length and subtlety of the breath
as an activity to be followed everyday.

vṛtya - to be surrounded or encompassed; to be abided or stayed or remainedprāṇa - vital force; the breath of life; breath; inward airvāyu - wind; air elementdīrgha - long, for a long time; a long vowelsūkṣma - subtle; atomic; intangibleca - andvṛtti - fluctuation; modification; turning; set in motion, course of action, behaviour, movement; activity, function; profession

Commentaries and Reflections

Commentary by Paul Harvey:

“You have to practice in such a way that
day to day the breath gets longer and subtler.”