Chö-ku, long-ku and trül-ku, exist as a singular field – the fourth, or indivisible sphere of being. This is known as dorje-ku (indestructible sphere) or ngo-wo-ku (essence sphere). Chö-ku, long-ku and trül-ku manifest as three inherent modes of access to the unitary experience of reality. But the experience of realisation sees no boundaries or divisions between them. Dorje-ku is the absolute undivided quality of enlightened nature. This is the sphere of complete freedom from inhibition – the sphere of indestructible reality, in which dualistic fixations have no ground.
Whatever arises in this condition becomes an ornament of the enlightened state, in the instant of its arising. From the pan-dimensional principle of tantra, in order to actualise this state – to become trül-ku – we need to unify emptiness and form though entering the sphere of energy.
p55-56, Wearing the Body of Visions, Ngakpa Chögyam, Aro Books, 1995, ISBN 1-898185-03-4