Kundalini Splendor

The Tantric interpretation of Kali standing on top of her husband is as follows: (taken from Wikipedia)

The Shiv tattava (Divine Consciousness as Shiva) is inactive, while the Shakti tattava (Divine Energy as Kali) is active. Shiva, or Mahadeva represents Brahman, the Absolute pure consciousness which is beyond all names, forms and activities. Kali, on the other hand, represents the potential (and manifested) energy responsible for all names, forms and activities. She is his Shakti, or creative power, and is seen as the substance behind the entire content of all consciousness. She can never exist apart from Shiva or act independently of him, i.e., Shakti, all the matter/energy of the universe, is not distinct from Shiva, or Brahman, but is rather the dynamic power of Brahman.[31]

Kali in Traditional Form, standing on Shiva's chest.

While this is an advanced concept in monistic Shaktism, it also agrees with the Nondual Trika philosophy of Kashmir, popularly known as Kashmir Shaivism and associated most famously with Abhinavagupta. There is a colloquial saying that "Shiva without Shakti is Shava" which means that without the power of action (Shakti) that is Mahakali (represented as the short "i" in Devanagari) Shiva (or consciousness itself) is inactive; Shava means corpse in Sanskrit and the play on words is that all Sanskrit consonants are assumed to be followed by a short letter "a" unless otherwise noted. The short letter "i" represents the female power or Shakti that activates Creation. This is often the explanation for why She is standing on Shiva, who is either Her husband and complement in Shaktism or the Supreme Godhead in Shaivism.

To properly understand this complex Tantric symbolism it is important to remember that the meaning behind Shiva and Kali does not stray from the non-dualistic parlance of Shankara or the Upanisads. According to both the Mahanirvana and Kularnava Tantras, there are two distinct ways of perceiving the same absolute reality. The first is a transcendental plane which is often described as static, yet infinite. It is here that there is no matter, there is no universe and only consciousness exists. This form of reality is known as Shiva, the absolute Sat-Chit-Ananda — existence, knowledge and bliss. The second is an active plane, an immanent plane, the plane of matter, of Maya, i.e., where the illusion of space-time and the appearance of an actual universe does exist. This form of reality is known as Kali or Shakti, and (in its entirety) is still specified as the same Absolute Sat-Chit-Ananda. It is here in this second plane that the universe (as we commonly know it) is experienced and is described by the Tantric seer as the play of Shakti, or God as Mother Kali.[32]

Kali and Bhairava (the terrible form of Shiva) in Union, 18th century, Nepal

From a Tantric perspective, when one meditates on reality at rest, as absolute pure consciousness (without the activities of creation, preservation or dissolution) one refers to this as Shiva or Brahman. When one meditates on reality as dynamic and creative, as the Absolute content of pure consciousness (with all the activities of creation, preservation or dissolution) one refers to it as Kali or Shakti. However, in either case the yogini or yogi is interested in one and the same reality — the only difference being in name and fluctuating aspects of appearance. It is this which is generally accepted as the meaning of Kali standing on the chest of Shiva.[31]