The fainting sensation and light-headedness arise due to the following reasons:
Pressurization of the carotid sinuses in the throat by the combined practices of ujjayi pranayama, khechari mudra and jalandhara bandha (in technique 2). The carotid sinuses are located in the neck beside the carotid artery which supplies the brain with its vital supply of oxygenated blood. These sinuses are sensitive to pressure and they regulate the heart rate and pressure and match it with respiration. They are vital in maintaining the oxygen balance of the body, among other things. During moorchha pranayama these sinuses are compressed continuously; this reduces the blood flow to the brain and directly induces fainting.
Kumbhaka further reduces the oxygen supply to the brain. This becomes especially noticeable if kumbhaka is held for a long time. The brain is hypoventilated. This leads to fainting.
These two influences, compression of the carotid sinuses and kumbhaka, supplement each other and anaesthetize the mind so that it becomes one-pointed. All mental fluctuations are stilled.
Moorchha pranayama is a very powerful practice especially if it is done with extended kumbhaka It relaxes the entire mind and body. It withdraws one’s awareness from the outside world (pratyahara) so that all external sounds, feelings, smells, etc. are cut off. Simultaneously, it renders the mind free of thoughts. This leads to meditation and bliss.