kapalbhati pranayama

When inhalation and exhalation are performed very quickly, like a pair of bellows of a blacksmith, it dries up all disorders from the excess of phlegm, and is known as Kapala Bhati (Hatha Pradipika 2:35).

The rapid breathing should be from the abdomen; the shoulders and face remain relaxed. The number of respirations can be increased from  the count of 10-50 (beginner) to 60-100 breaths per round (advanced practitioner). 

This is a good practice for respiratory disorders cause has a cleansing effect on the lungs, it balances and strengthen the nervous system and tones the digestive organs, it purifies the nadis and removes sensory distractions, it energizes the mind for mental work and remove sleepiness.

In some texts is not indicated like a pranayama technique, but only like a cleansing technique.