What else could there be to let go of after seeing through the illusion of the witness? It is going into the opposite direction. Instead of driving any trace of self to the vanishing point, you come to see everything as mind stuff, emanation of mind. The unitive experience gets you part of the way there, but there is still a sense of a self looking out at the world, albeit a world that has no inside or outside.
The nondual view goes further than the unitive experience in seeing that everything that is experienced, including emotions, memories, concepts, the void, etc. are all manifestation of mind, like waves on water. Thus, everything takes a shade of immateriality, impermanence, both self and non-self at once. The illusion of an ego that needs to be defended can be seen through and held lightly.
Like the other types of awakening, there seems to be several paths leading to that view. For me it happened when turning my attention inward during mindfulness of mind (or mahamudra) meditation as described in TMI11, which has become one of my main practices. I sometimes meditate with my eyes closed, so internal sensations dominate, such as feelings of the body, ideas, fleeting thoughts, and emotions. After some time meditating the mind states slow down enough to be able to carefully notice each perception as it arises, evolves, and passes away.
One day it became clear that all perceptions arise from the same substrate, and that they are self-perceiving, just like a flame is self-luminous and does not need an external source of light to illuminate it. Perceptions do not arise in the mind so much as they are forms that the mind takes and are not separate from the mind. It is very much like waves on the ocean. Are the waves that occur in the ocean separate from the ocean? Or are they of the same nature as the ocean, just one of its many forms? Whereas letting go of the witness is the seeing through of an illusion, breakthrough into the nondual is a unification step. It is the realization that everything experienced, consciously and unconsciously, has the same “taste”, the same nature, that of mind stuff.
The effect of seeing this clearly, of internalizing this nondual view, is that internal states take up an even more unsubstantial feel than before, lose their solidity. There is even more acceptance of all states, all emotions, all feelings, as they arise and pass away, like the weather. From the nondual perspective, it is seen that there are no “bad” or “good” states of mind, no dukkha and no awakening, no separation between inner and outer. Everything that happens is accepted as is. Outer universe and internal mind, it is all one continuum. On the mind aspect, it is all mind stuff. Sure, there is an objective universe “out there” made of atoms and energy, but our minds will never have direct access to it. That objective universe unfolds according to the laws of physics, following causes and conditions, even though the web of interconnectedness is far too complex for any mind to ever follow it. Each of our minds represents a small corner of the universe, what it has access to, and superimposes its own interpretations, classifications, value judgements, etc. All we will ever have access to is some part of the mind that can be made conscious, but in this nondual view the mind is seen an integral part of the world, so itself subject to the same causal unfolding of the universe.
The best expression of the vacillation between being nothing and being everything that I ever came across was by Sri Nisargadatta Maharaj. He said, “Wisdom is knowing I am nothing, Love is knowing I am everything, and between the two my life flows.”
When you read the interpretation that the sages make about the nondual, there is almost always a note of supernatural concepts, e.g. that this mind within which everything happens is universal, shared among all sentient beings. It certainly is the way it feels, that this mind is impersonal and universal, but that does not mean it is actually that way. As a naturalist (someone who does not believe in supernatural concepts), I still hold that each mind is private.