Yoga Sutra 1.34 expulsion and retention of prana
Sūtra I.34 Or by expulsion and retention of prāṇa Expulsion is emission of the abdominal air through the two nostrils by a conscious effort; retention refers to the process of prāṇāyāma. By these two means one can attain steadiness of the mind. The word Or means an alternative, so this is a means to steadiness…