Vijnana Bhairava Tantra
21: Just as by means of the light of a lamp, and the rays of the Sun, portions of space etc., are known, even so, Oh dear one, by means of Sakti is Siva (who is one's own essential Self) cognized.
22: Sri Devi said: O Lord of the Gods, who bears the trident and skulls as ornaments, tell me how can that Supreme Goddess, the Highest Sakti who transcends all notions of direction, space and time and all manner of description be known?
23: By what means can that complete state of Bhairava which is full of the bliss of non-difference from the universe be realized? In what way is the paradevi, the Highest Sakti said to be the door of entrance into Bhairava? Please tell me in the Vaikhari form (in human language) that which I know fully well at the para level or please instruct me in such a manner that I may understand it fully.
24: Bhairava says: Para devi or Highest Sakti who is of the nature of visarga (creation) goes on ceaselessly expressing herself upward (urdhve) from the centre of the body to dvadasanta or a distance of twelve fingers in the form of exhalation (prana) and downward (adhah) from dvadasanta to the centre of the body in the form of inhalation (jiva or apana). By steady fixation of the mind at the two places of their origin (viz., centre of the body in the case of prana and dvadasanta in the case of apana), there is the situation of plenitude which is the state of parasakti or nature of Bhairava.
25: Of the breath (exhalation or prana) arising from the inner i.e. the centre of the body (hrt) there is non-return for a split second from the dvadasanta (a distance of twelve fingers from the nose in the outer space), and of the breath (inhalation or apana) arising from dvadasanta i. e. the outer space, there is non-return for a split second from the centre of the body (hrt).l If one fixes his mind steadily at these two points of pause, one will find that Bhairavi the essential form of Bhairava is manifested at those two points.