Thanissaro Bhikkhu, The Paradox of Becoming:
Second, mastery of jhana
provides long periods of mental stillness that enableone to observe how passion
and delight can form a location of becoming around
the focal point at the heart of jhana itself. To observe this focal point—rather
than simply being absorbed in it—one must step back a bit from one’s full
absorption without yet destroying the jhana. The ability to do this relies on two
things: the fact that consciousness can serve as food for consciousness, and that
jhana provides an expanded, whole-body awareness. One observes a state of
jhana consciousness as one’s food, while inhabiting another locus of
consciousness within the expanded field of awareness provided by that jhana
consciousness. This, however, is a special skill, developed above and beyond the
four jhanas themselves.
The Blessed One said: “Now what, monks, is five-factored noble right
concentration? There is the case where a monk … enters and remains in
the first jhana … the second jhana … the third jhana … the fourth jhana
….
“And furthermore, the monk has his theme of reflection well in hand,
well attended to, well-considered, well-tuned/well-penetrated by means
of discernment.
“Just as if one person were to reflect on another, or a standing person
were to reflect on a sitting person, or a sitting person were to reflect on a
person lying down; even so, monks, the monk has his theme of reflection
well in hand, well attended to, well-pondered, well-tuned/well-penetrated
by means of discernment. This is the fifth development of the five-
factored noble right concentration.” — AN 5:28
In this fifth factor, one can observe each jhana while still in the expanded
range it provides. One can watch not only the focal point of awareness, but also
all the other mental factors that go into making the jhana.
My understanding of how one would approach this, in brief:
1) Concentrate (pay attention to the breath) until there is an experience of intense stillness. (The particular jhana doesn't matter, but I have found that 2 and 4 work best for me.) This stillness is independent of the attention wave, vibrations, etc. It should be experienced as if the mind is paralyzed in some way, or as if you are having the paradoxical experience of a PCE or PCE-like experience with the attention wave superimposed on it.
2) You can "be" the stillness. To do this, you have to approach it in the opposite way that MCTB suggests approaching Equanimity in order to attain MCTB 1st path. Let the "stillness / PCE-like experience" be in the background. Don't include it in awareness. Don't look at it. Keep your back to it. This allows you to inhabit / feed on an affective or shadow copy of the stillness you generated.
3) From this vantage point of "stillness", observe all the other stuff at once (vibratory and sensory) with absolute disinterest, and with the understanding that it can pass away. ("What is, what has come to be, that I abandon.")
Doing this makes pieces of the "symbolic overlay" (non-actual experience) fall away spontaneously. The magnitude of this seems to depend on how concentrated you are. In context of extreme concentration, it's like witnessing the Second Coming of Christ. In context of moderate concentration, it's like witnessing a minor miracle, such as water being transmuted into wine. In context of weak concentration, the effect can be discerned, but is quite minor.
What defines the strength of your concentration is 1) how PCE-like you can make your jhanic experience in terms of generating "stillness", and 2) your ability to sustain that over long periods of time. Nothing else. Not the conventional MCTB criteria.
If your concentration was incredibly high, and you could sustain it as long as you wanted to, perhaps you would get fully enlightened in a week, like the suttas say. That is how profound the perceptual shift is that this practice engenders. Perhaps that is why the suttas emphasize jhana so much.
Don't expect any warning that the symbolic overlay will fall away, it seems to be quite spontaneous.
When the symbolic overlay falls away, it seems to disrupt concentration (at least for me), so be prepared for that, and just re-create the stillness, and repeat.
Now, what I explained is not what Thanissaro Bhikkhu has described, as it appears that he has described the opposite (inhabit something that isn't the stillness, watch the affective / shadow stillness pass away). So, master the practice I described, and then reverse the background / foreground: an alternative application of the "fifth factor". However, this is probably only suitable as an advanced practice, so consider sticking with the "normal' background / foreground as long as it seems helpful to. (I haven't tried it.)
How to concentrate in the appropriate way? Relax, attend to the breath without "trying", don't aggravate the attention wave. If you approach this like MCTB jhana you probably won't get it. But, no explanation beyond this will be as helpful as experimenting for yourself.
Note that, unfortunately, the appropriate level of concentration is easier to attain on retreat, or at least when it's one's sole formal practice (and one has many hours per day to dedicate to it).