Bellows is an instrument used by the blacksmith to reinforce the fire by quick supply of air. Therefore, quick inhalation and exhalation is called bellows breathing.
Adopting a comfortable posture, shield the nostril with your palm. Without pausing exhale and inhale in quick succession (10 to 20). Exhale fully and hold the breath outside. Now inhale through the right nostril and hold the breath inside. Repeat with quick exhalations and inhalations (holding of the breath is optional).
In subtle bellows breathing, the breathing is very soft.