Why ChinMudra? = Vajrasanais about collecting the disparate energies or prana into the person. Prana flows within and outside the body. An open palm would mean that the prana which moves to the extremities is unable to return which makes the flow unidirectional. However, with chin mudra, this loop is closed, like any good electrical circuit, resulting in uninterrupted flow of prana around the person, increasing retention and charge of prana within the system.
This asana is considered to be one of the 32 most important asanas by all ancient texts.