Yoga sutra 2.50
bāhya-ābhyantara-stambha-vṛttiḥ-deśa-kāla-saṁkhyābhiḥnparidṛṣṭaḥ dīrgha-sūkṣmaḥ
By observing the actions of exhalation, inhalation, and breath lock, whilst focusing on place, duration and numbers, to produce a long smooth breath.
The description of prāṇāyāma begins with exhalation (bāhya vṛttiḥ) which is purifying. Then, inhalation (ābhyantara vṛttiḥ) which is stimulating nature or ‘with nature’, and holding your breath (stambhavṛttiḥ) to extend the life-force within you. Technical elements include placing the attention in a specific place (deśa), determining how long the breath will be (kāla) and keeping track of the rounds of breathing as you do it (saṁkhyā).
For a detailed guide on the practice of prāṇāyāma see: So what is pranayama? (pranavashyauk.com)
Damian Cadman-Jones is an authorised teacher of Prana Vashya Yoga™