Kapālabhātī prāṇāyama is considered as a penance. Kapāla means skull or cranium and bhāti means appearance (looking clean). When this prāṇāyama is practiced regularly, one can easily enter into trance and kuṇḍalinī also gets activated. This prāṇāyama is different from bhastrikā prāṇāyama. The main purpose of kapālabhātī prāṇāyama is to cleanse the cranium and make it shine. During this prāṇāyama almost every part of the brain gets cleansed and makes the movement of kuṇḍalinī easier from ājñācakra to sahasrāra. Some call this prāṇāyama as part of kriyā yoga. Nāḍi śodhana prāṇāyāma should be practiced only after practicing this prāṇāyama.
This is easy to practice. This should be practiced in empty stomach. Sit in a comfortable position and place the palms facing down on the thighs leaving adequate gap in the armpits, which is very important to avoid frozen shoulders. Spine should be erect and head slightly tilted backwards. Inhale using yogic breathing (expanding abdomen) and exhale with force. Both inhalation and exhalation should be done through both the nostrils. Inhalation should be deep and exhalation should be with force. While exhaling, abdomen should be contracted, which is part of yogic breathing. Too much of force during exhalation should not be used in the beginning. The force can be increased over a period of time. During practice, attention should be fixed at ājñācakra and observe the vibrations there.
In the first stage, five rounds can be practiced consecutively and in the second stage, ten rounds can be practiced. One round means one inhalation and one exhalation. After five rounds (five inhalations and five exhalations) pause the practice for some time and then resume. Maximum permissible is only 15 rounds at a time. If one develops headache or other discomforts, this should not be continued.
This prāṇāyāma cleanses the cranium, lungs; purifies blood circulation, nervous system and lymphatic system. This also improves digestion. It induces bliss, inexplicable happiness.
Nāḍi Śodhana Prāṇāyāma can be read in this link.