One way in which tantric Advaita differs from Advaita of Adi Shankaracharya is that the tantras don’t deny the reality of the world. But neither do they say that the world is real in itself, as an un-illumined person experiences it. No, it is real because it is the dance of Shiva, who is Reality itself, or because it is the dance of the Divine Mother who is one with Shiva, or because it is the dance of the Divine Mother who herself is both the nitya and the lila.
Speaking of God as the nitya and the lila, Sri Ramakrishna says: ‘Water is water whether it is still or in waves. The snake is a snake whether it is coiled up motionless or wriggles along. A man is the same man whether sitting still or engaged in action.’
Such statements are closer to tantric Advaita than to Vishishtadvaita because they identify the universe with Shakti, which is in turn identified with the Absolute. And they are closer to tantric Advaita than to Shankara Advaita because they speak of the reality of the world as a manifestation of Shakti.
One might point out that none of this necessarily contradicts traditional Advaita. No, it does not, and that’s important to recognize. The difference we are pointing to is one more of emphasis, not of absolute difference. The Upanishads themselves say: ‘All this is verily Brahman.’
Advaitins also accept that, but tantra makes a special point of it, and says that therefore the world is not unreal. Instead, traditional Advaita Vedanta emphasizes that ‘Brahman is true, the universe is false’, based on its definition of truth.
In confrontation with each other as opposing philosophical schools, they harden their respective positions into absolutes. And thereby the difference is stark. If, however, we soften our philosophical stance, we can see that the difference is more one of emphasis. And that is how the great Acharya Shankara could be both an Advaita Vedantin and a tantric.