Yoga Sutra 2.51 the fourth pranayama
Sūtra II.51 The fourth prāṇāyāma comes when both external and internal fields have been felt into The field of the external operation, as measured in terms of place, time, and number, has been practised and felt into. (The second prāṇāyāma) was practice in feeling into the field of the internal operation, as measured similarly. In…