The Master's constant formula, in a sense their essential 'teaching', taking any and every dharma and declaring that it 'neither is nor is not', means precisely (and factually) that it 'is neither positive nor negative'. Therefore it is idle to do what we are apt to do, that is immediately to look for that which it ('really', as we say) is - since we are begging the question, having just been told that it IS not.
That which is not positive and not negative is the result of the mutual extinction, or negation, of each (Shen Hui's double negative), by means of which each characteristic is cancelled by its counterpart (as light by shade, and shade by light, in positive and negative films), leaving a phenomenal blank, no phenomena whatever, that is perfect objective voidness, unhappily, even absurdly, called 'The Void'.
Shen Hui has stated that, to the awakened, voidness no longer is such - which means that voidness no longer appears as an object. But that which, viewed objectively, is void can never be anything else, can never, for instance, be 'full', a 'plenum', as has been maintained (but never, I think, by a Master): that whose identity is voidness of objects can never not be void of objects without ceasing to be what it is. As long as it is itself an object, it must remain devoid of objects, but when it ceases to be an object, ceases to be itself at all, it thereby returns to subject, as which it is pure potential, and, as such, a potential plenum.
That, no doubt, is the sense of Shen Hui's statement, which has caused some disturbance in the heads of the scholars.
Note: May we not generalise from this and declare that the same applies to all objects? Is it not evident that every object, when it ceases to be itself, i.e. objective, thereby becomes void, returns to subject and re-becomes potentiality - which is all that anything IS? Always bearing in mind that 'potentiality' is only a pointing, not any 'thing', for phenomenally it must ever be total absence, which non-objectively must be total presence, just as what objectively is void, subjectively is a plenum.