Bhagavad Gītā Chapter Five verse 27

sparśān kṛtvā bahir bāhyāṃś cakṣuścaivāntare bhruvoḥ |
prāṇāpānāu samāu kṛtvā nāsābhyantaracāriṇāu ||

samā - equal, constantprāṇa - breath, inward air, vital forceapāna - downward air

Commentaries and Reflections

T Krishnamacharya:

“Use Samavṛtti in Prāṇāyāma to reduce the power of Rajas and Tamas.”