For my part, I've self-induced an OBE, or otherwise known in psychology as a psychotic break, as if one is watching onesself in a movie. Only once, not through drugs but self-scrutiny, it was an unpleasant experience, and I think I understand the phenomenon.
We don't see everything at once. We have, as part of our awareness, a mental picture of the immediate environment along with extremely short term memory. The first is an internal map of our surroundings to put sound in a direction and context, the latter is to establish a sequence of events in this environment.
The problem is all of this is otherwise served up indifferently; memories are tagged but not separated in how they are replayed. If a very short term memory gets tagged incorrectly as a long-term memory, it becomes deja-vu. Similarly, if a direct experience gets tagged as a related experience, one has the viewpoint outside one's body, the body image substituted for one's own perspective. In OBE and psychotic breaks, immediate experience is tagged as related experience, and one sees their own body, either on the table or going through other motions of life seemingly external to consciousness.
The first half of the Near-Death experience would seem to be similar, in that memory tagging breaks down before the player and the result is an artificial OBE, not a real one; one does not come back with any hidden information not in the mental picture of the immediate environment.
The second half, the tunnel, is the latter stages of brain shutdown, as first the retinas (total darkness) and then the optical cortex (sensation of light) are starved of oxygen.
The brain is apparently not a completely volatile memory system, as some people do have a memory of these experiences.
