Yōga Sūtra 1.34

प्रच्छर्दनविधारणाभ्यां वा प्राणस्य।

pracchardanavidhāraṇābhyāṁ vā prāṇasya।

[प्रच्छर्दन विधारणाभ्यां वा प्राणस्य।
pracchardana = exhalation; vidhāraṇābhyāṁ = by retention; vā = also; prāṇasya = of breath;]

Also, by exhalation and retention of breath.

The specific breathing exercise (Prānāyāma) to be practiced is described here. Focus needs to be on complete exhalation of breath, and then, post inhalation, to retain the breath as long as possible. It is further elaborated in next section, where importance of slowing the breath as much as possible, and during retention focusing the mind on one principle is recommended. Patanjali does not advocate any breathing exercises that involve fast breathing.