The practice of pranayama develops a steady mind, strong willpower, and sound judgement.
- BKS Iyengar

Tasmin sati svasa prasvasaho gati-viccheda pranayama

Pranayama is the conscious, deliberate regulation of the incoming and outgoing flow of breath replacing unconscious patterns of breathing. It is possible only after a reasonable mastery of asana practice.
- Patanjali Yoga Sutra 2.49

Bahya abhyantara stambha vritti desa kala samkhyabhih paridrstah dirgha suksmah

Pranayama has 3 movements inhalation, exhalation, and retention, prolonged and fine; the regulation of these processes is achieved by modulating their length and maintaining this modulation for a period of time, as well as directing the mind into the process in uniform fashion.
- Patanjali Yoga Sutra 2.50

Bahya abhyantara visaya aksepi caturthah

The fourth type of pranayama transcends the external and internal pranayamas, and appears effortless and non-deliberate. An entirely different state of breathing appears in the state of yoga. Then the breath transcends the level of the consciousness.
- Patanjali Yoga Sutra 2.51

Tatah ksiyate prakasa avaranam

The regular practice of pranayama reduces the obstacles that inhibit clear perception and removes the veil covering the light of knowledge and wisdom.
- Patanjali Yoga Sutra 2.52

dharanasu ca yogyata manasah

The mind becomes fit for concentration. And the mind is now prepared for the process of direction toward a chosen goal.
- Patanjali Yoga Sutra 2.53