Bhastrika is a form of yogic practice. Bhastra also means a pair of bellows or a leather bag for holding water. In Yoga, bhastra and bhastrika are techniques of breath control by the movement of the abdomen, like the bellows. This is a specially preferred variety for the purpose of quick arousal of the dormant spiritual power called kundalini.
Bhastrika may be divided into two parts. The first part consists of incessant abdominal breathing in which there are inspirations and expirations in quick succession with a conspicuous and rather forceful movement of the abdominal wall back and forth. The backward movement is effected by a sudden contraction of the front abdominal muscles. When the abdominal wall is relaxed, it comes forward passively.
After twenty or thirty such kapalabhati movements, like a pair of bellows, the second part begins. In it there is a puraka (deep inspiration) through the right nostril, then kumbhaka (holding of breath inside), followed by recaka (a deep expiration) through the left nostril.
The two parts together make on round of Bhastrika.
Several such rounds are practiced in a sitting. It is one of the six cleansing acts of hatha yoga.
A special feature of this first part is that in it the lung ventilation is increased more than then times and thus carbon dioxide is removed very effectively, which makes the respiratory center rather silent, and the following kumbhaka can be continued more easily for a longer time. This has great value in pranayama for the arousal of kundalini.
Source – Yogic Pranayama (1983) by K S Joshi – Orient Paperbacks Delhi
Rajayoga, Concentration and Meditation (1970) K S Joshi – Satya Press, Puducherry.
Encyclopedia of Hinduism - Volume 2 - page 281