The essential understanding is that in reality nothing is. This is so obvious that it is not perceived. We quote Hui Neng's 'From the beginning not a thing is' without apprehending its full significance. We refer to the Void and Emptiness without realising what is implied. What is meant is just what is said, i.e. that nothing is - that Nothing alone is what is, not that no thing is real in Something, not that in positive Being, which we tacitly assume, no object is real. Positive Being is not to be assumed, but negative Being - Non-being. It is non-being only that is, and there is nothing but that. It is only in function of Non-being that being seems to be.
Non-being is; and it is because Non-being is, and only Non-being, that any being can be, for being is a manifestation of Non-being.
The Realisation of the Void
Being must be replaced by Non-being - in order that anything may be.
For Being is a projection of Non-being.
This is the necessary realisation, and, perhaps, the ultimate - that Nothing alone is, that there is only Is-not, that Is is only by virtue of Is-not - for nothing but Nothing is of itself, and only the isness of Is-not can be.
That is the realisation of the Void.
Note: This is the deep meaning of the Buddha's words in the Diamond Sutra about Bodhisattvas knowing no Being, and of Hui Neng's declaration that we must rid ourselves of the idea of Being as well as of self. Its meaning has been masked by the article 'a' usually given it elsewhere, which appears to assimilate it to a term for self, ego, individual, whereas it is to be applied to the plane of 'reality'.
The Void (Non-Being) alone is complete.