Yoga Sutra 2.49 cut off the flow of in-breath and out-breath
Prānāyāma is to sit in the posture and cut off the flow of in-breath and out-breath
The posture has been mastered. Now when he inhales outside air, it is called the in-breath (śvāsa); when he exhales the air from within, that is the out-breath (praśvāsa). Cutting off the flow of these two, so that they both cease, is prāṇāyāma.
He is now ready for prāṇāyāma. Prāṇāyāma is to sit in the posture and cut off the flow of in-breath and out-breath. When sitting in that posture made quite firm, he inhales outside air, it is called the in-breath (śvāsa). As water is sucked up through a tube by a continuous action, so by a continuous action through the two tubes of the nostrils, the external air is drawn in, in association with the downward-going current (apāna), and this drawing in is called the in-breath. Then when he exhales the air from within, in association with the operation of the upward-going current (prāṇa), its going out is the out-breath. Thus the two flows are distinguished. What then? He continues: cutting off the flow of these two is called prāṇāyāma. Of this –