"The Heart, says Abhinavagupta, is the very Self of Siva, of Bhairava, and of the Devi, the Goddess who is inseparable from Siva. Indeed, the Heart is the site of their union (yamala), of their embrace (samghatta). This abode is pure consciousness (caitanya) as well as unlimited bliss (ananda). As consciousness the Heart is the unbounded, infinite light (prakasa) as well as the freedom (svatantrya) and spontaneity (vimarsa) of that light to appear in a multitude and variety of forms. The Heart, says Abhinavagupta, is the sacred fire-pit of Bhairava.
The Heart is the Ultimate (anuttara) which is both utterly transcendent to (visvottirna) and yet totally immanent in (visvamaya) all created things. It is the ultimate essence (sara). Thus, the Heart embodies the paradoxical nature of Siva and is therefore a place of astonishment (camatkara), sheer wonder (vismaya), and ineffable mystery. The Heart is the fullness and unboundedness of Siva (purnatva), the plenum of being that overflows continuously into manifestation. At the same time, it is also an inconceivable emptiness (sunyatisunya). The Heart is the unbounded and universal Self (purnahanta).
The Heart of Siva is not a static or inert absolute, however. In fact, the non-dual Kashmir Shaiva tradition considers it to be in a state of perpetual movement, a state of vibration (spanda) in which it is continuously contracting and expanding (samkoca-vikasa), opening and closing (unmesa-nimesa), trembling (ullasita), quivering (sphurita), throbbing, waving, and sparkling (ucchalata). The intensity and speed of this movement is such that paradoxically it is simultaneously a perfect dynamic stillness.
The tradition states that the Heart is the enormous ocean (ambunidhi), the ocean of light, the ocean of consciousness. The waters of consciousness that in man are broken by countless polarizing and divisive waves (urmi) may be easily brought to a state of dynamic stillness by the process of immersion or absorption (samavesa) in the Heart.
The perpetual effervescence, ebullition, and incandescence at the center of Siva's being provides the motor, as it were, that drives the entire process of manifestation; or, as Abhinavagupta terms it, of emission of the entire universe. Equally important, through the medium of the empowered mantra, the power that resides in the Heart provides the possibility of a return to Siva, which is to say, the possibility of enlightenment. Thus, this power in the Heart, which is the Goddess, is at once centrifugal (pravrtti), that is, emissional and expansive, as well as centripetal (nivrtti), that is, absorptive and unitive. This centripetal motion of the Goddess resides in man as the kundalini energy that impels one back to the source and center of one's being in a completely natural and spontaneous (sahaja) way."
~ Paul Muller-Ortega, from The Triadic Heart of Siva