Jalandhara Bandha (Chin Lock)
On both sides of the neck, carotid arteries bring blood supply to the brain. Applying a chin lock causes pressure to the arteries and the nerve impulses begin to travel to the brain directly effecting body consciousness.
Chin Lock stimulation can slow down the heart and, therefore, vijnani nadi (channel of consciousness) can be brought under conscious control. Shiva Samhita states that a blissful state of mind can be experienced with the pressure on the carotid sinus nerves. Applying a chin lock can make it easier to practice inhalation and exhalation hence regulating vijnani nadi becomes easy.
When practicing khumbhaka (breath retention), practicing proper jalandhara bandha becomes important. Improper practice can cause the air to rush out the auditory tubes, disturb the inner ear and cause various disorders. Closing the glottis first; then, proper application of jalandhara bandha, makes khumbhaka easy.
Practicing jalandhara bandha can help attain conscious control of vijnani nadi and the state of joy before meditating.
(Science of Breath, 97)
Note: the practice of bandhas and khumbhaka is for advanced yoga practitioners. This practice has prerequisites and counter indications and should be done under a guided supervision of a qualified teacher.
PS: the image is from google search and does not belong to Manasa.