Yoga Sutra 2.51 the fourth pranayama
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 both practices, the breath became long and fine. The fourth prāṇāyāma comes after the stages (mild, medium and intense) of these two practices have been gradually mastered, and it consists of cessation of both the operations. Whereas the third prāṇāyāma was stopping the breath without having previously brought to awareness the fields (of external and internal objects), the breath becoming long and fine simply by this practice according to place, time, and number; but the cessation in the fourth one comes only after having already brought to awareness those fields, feeling into them by gradually mastering the stages. This is what distinguishes the fourth prāṇāyāma.
The fourth prāṇāyāma comes when both external and internal fields have been felt into. The field of the external operation is the toes, etc., for the external air being drawn in, is felt pervading the inner regions. The field of the internal operation is earth and the other elements. The expiration outwards is felt pervading the earth and the other elements, as practised in terms of place, time and number. This fourth prāṇāyāma comes when both external and internal fields have been felt into.
The (first) practice was feeling into the pervasion of the field which is the object of the external operation, as measured by place, time and number; the second was feeling into the pervasion of the field which is the object of the internal operation, similarly. In both practices, the breath became long and fine. By feeling into the fields of the external and the internal objects, length and fineness of the breath come about, in both of them.
What follows? After the stages (mild, medium, and intense) have been gradually mastered, corresponding to the feeling into both operations of the breath, there is cessation of the flow of breath in both the external and internal operations, the flow of prāṇa (the upward and outward current) and apāna (the downward current).
(Opponent) The third and fourth prāṇāyāma-s are the same; one finds no distinction between the two.
(Answer) In the third one there is no prevous bringing to awareness of the fields; it is cessation of the breath by a single effort at cutting off the course of the breath. The breath becomes long and fine by practising this alone, according to time, place and number. But the cessation in the fourth one comes after bringing to experience the fields of exhalation and inhalation taken to the very limit, gradually mastering the stages, and this is the distinction between the third and the fourth.