What Goenka describes seems to be common on his retreats (the body feels like it's dissolving is how I'd describe it). Scared me a bit the first time it happened.
However, in the Visuddhimagga map this comes after some other rather significant insights:
1. Analytical Knowledge of Body and Mind (nāma-rūpa-pariccheda-ñāna)
2. Knowledge by Discerning Conditionality (paccaya-pariggaha-ñāna)
3. Knowledge by Comprehension (sammasana-ñāna)
4. Knowledge of Arising and Passing Away (udayabbaya-ñāna)
So it seems odd that it's a reasonably early occurrence. But then Goenka's approach is a bit different from the Mahasi school, so maybe things happen in different orders.
Anyway, from Mahasi Sayadaw: Progress of Insight
http://aimwell.org/Books/Mahasi/Progres ... issolution" onclick="window.open(this.href);return false;
And, of course, you can read about it in the Visuddhimagga.5. Knowledge of Dissolution
Noticing the bodily and mental processes as they arise, he sees them part by part, link by link, piece by piece, fraction by fraction: "Just now it arises, just now it dissolves." When that knowledge of arising and passing away becomes mature, keen and strong, it will arise easily and proceed uninterruptedly as if borne onward of itself; also the bodily and mental processes will be easily discernible. When keen knowledge thus carries on and formations are easily discernible, then neither the arising of each bodily and mental process, nor its middle phase called "presence," nor the continuity of bodily and mental processes called "occurrence as unbroken flux" is apparent to him; nor are the shape of the hand, the foot, the face, the body, and so on, apparent to him. But what is apparent to him is only the ceasing of bodily and mental processes, called "vanishing," or "passing away," or "dissolution."
For instance, while noticing the rising movement of the abdomen, neither its initial nor middle phase is apparent, but only the ceasing or vanishing, which is called the final phase, is apparent; and so it is also with the falling movement of the abdomen. Again, in the case of bending an arm or leg, while noticing the act of bending, neither the initial nor the middle phase of bending is apparent, nor is the form of the limb apparent, but only the final phase of ceasing and vanishing is apparent. It is similar in the other cases of stretching a limb, and so on.
Metta
Mike