When we sit in meditation we do not see ourselves in an unbroken descent, constantly falling into our present embodied state of conditionality in which we are this phenomenal being or function (用) having lost sight of essence (體). With this loss of essence or substance of mind we are overwhelmed with phenomenality. Nothing we behold is other than conditioned—a function. Even as we sit we have no way of going beyond this dense, painful layer of phenomenality.
Zen wants us to reverse this fall; to return to essence or the substance of mind (which I generally refer to as pure Mind). While this sounds simple enough, sitting in meditation we can't see this essence. We only witness phenomena such as our thoughts and emotions or bodily sensations. We don't notice anything else. Yet, according to Zen there is something more we are missing during our introspection. However, its actual purity and unconditionedness almost guarantee we will not encounter it face to face without a difficult struggle. At best, we will only be able to envisage it as a dim abstraction, further reifying this abstraction, which is not the way to go. Despite such a warning we continue. We even imagine the only escape from this is doing lots of zazen.
But doing zazen can quickly become a dead-end because the practice is too focused on the physical side of zazen, the "za" side. Here, too, does the adept receive little or no instruction as how to proceed further. It is generally, just be aware of the rising and falling of your thoughts. Almost always ignored, is that the adept should contemplate what is prior to the arising of thought and what does not cease with the ceasing of thought. With gnosis of this the adept all at once sees essence which is beyond the sphere of the mundane.
With this realization, now, during meditation, our previous world of birth and death is seen to be an illusion which depends upon essence which we have uncovered. The illusory world—even this body and thoughts of ours—is the function of essence whereby essence returns to itself by penetrating, thoroughly, through the function/illusion. At the instant of penetration (samadhi) we always behold essence thus becoming more and more of it hence, leaving the former illusory world behind in the sense of no longer desiring it. This is also the perfection of wisdom (prajñāpāramitā).