Sutra 2.49 says: Tasmin satisvasa prasvasayoh gativicchedah pranayamah Tasmin –
on this sati – being accomplished svasa – inbreath, inhalation prasvasayoh – outbreath, exhalation gati – movement, motion, path vicchedah – cessation, stop pagepranayamah – regulation. Pranayama is the regulation of the incoming and outgoing flow of breath with retention.
It is to be practiced only after perfection in asana is achieved. It’s more then just breathing friends.