School of Yoga explains kapālabhātī – (kapāla = skull + bhātī= shining)

In kapālabhātī-prāṇāyāma, the inhalation is passive, but the exhalation is active using the abdominal muscles.

School of Yoga explains – kapālabhātī-prāṇāyāma technique: 

• Sit in padmāsana, siddhāsana or vajrāsana.
• Close the mouth. Let the tongue touch the top of the palate to close the circuit of prāṇa flow.
• Place hands in chin or chinmaya-mudra.
• Breathing in naturally, let the abdomen relax without effort or exertion.
• At the end of inhalation, push air out vigorously using the abdominal muscles, .
• As a result of the pressured exhalation, there should be a feeling of slight light-headedness.
• Also, one should experience tightening of the anus due to forced exhalation.
• Repeat 20 to 50 times spread over a maximum of 2 sittings.

School of Yoga explains – kapālabhātī benefits: 

• This prāṇāyāma strengthens the digestive system and lower respiratory tract. Multiple digestive ailments such as acidity, flatulence, constipation, piles, diabetes and other lung illnesses such as asthma, and emphysema are improved by this exercise.
• The rapid movement of air increases volumetric efficiency of the lungs and heart function. The rapid exhalation through the nose results in phlegm, mucous and other secretions being ejected from the windpipe.
• This is especially useful in managing abdominal illnesses such as diabetes, gastritis, hyper-acidity, kidney and liver ailments etc.
• Due to the rapid intake of the abdominal muscles, abdominal organs move and massage spine creating a shock effect which results in transmission of the shock to the brain. This could result is slight light-headedness.
• Additionally, this energises and stabilizes the central and peripheral nervous system, the sympathetic and para-sympathetic nervous system, especially the spinal nerves. 
• Since the spinal chord is energised, all the main nāḍi get energized also.

Hatha Yoga Pradeepika on kapālabhātī  (Chapter 2, verse 35 to 36)

• Kapālabhātī – when inhalation and exhalation are performed rapidly like the bellows of a blacksmith. This is called kapālabhātī which overcomes orders of phlegm.
• When one is freed from obesity and phlegm by practicing the 6 acts and subsequently practices prāṇāyāma, the practitioner will achieve success without difficulty.

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                 Kapālabhātī-prāṇāyāma technique