Pranayama is the fourth limb of Patanjali‘s Raja Yoga. The premise: breath is the most accessible face of Prāṇa, the life-force; regulating one regulates the other.
Techniques range from simple ratio breathing (lengthening exhalation) to advanced methods like nadi shodhana (alternate nostril), kapalabhati (cleansing breath), and bhastrika (bellows). Classical texts warn that pranayama without preparation and a teacher can be destabilizing — it works directly on the nervous system.