Instruction in pranayama can begin after one has learned the asanas well and can practice them with ease (Jois 2002 23).
“Pranayamameans taking in the subtle power of the vital wind through rechaka [exhalation], puraka [inhalation], and kumbhaka [breath retention]. Only these kriyas, practiced in conjunction with the three bandhas [muscle contractions, or locks] and in accordance with the rules, can be called pranayama” (Jois 2002 23). The three bandhas are “mula bandha, uddiyana bandha, and jalandhara bandha, and they should be performed while practicing asana and the like” (Jois 2002 23). “When mula bandha is perfect, mind control is automatic” (“Ashtanga Yoga”). “In this way did Patanjali start Yoga. By using mulabandha and by controlling the mind, he gradually gained knowledge of Yoga” (Jois 2003 11).
[Cite: Source Ashtangayoga.com http://www.ashtanga.com/html/background.html ]