Matt:
Thanks - that brings up a related question that I've been pondering.
My original post was perhaps not as clear as it could have been, or alternatively maybe I'm not as clear about what I'm doing as I think I am. :-)
The 'merging with the jhana' thing is something that I found a way to do back in 2017 or so, but for whatever reason didn't play around with too much at the time.
More recently, in my insight practice (which is based around open awareness and gentle investigation - I used to be more of a 'shredding' type like neko describes, but in the last few years the gentle approach has resonated better with me whereas very intense practices have tended to feed some less helpful patterns in myself), I've been exploring various ways in which I seem to be 'holding on' to things. One day I found that I momentarily 'let go' of something and experienced something like a 'merging' with the field of awareness, but as it often does, my mind recoiled in horror and refused to go back there again.
So over the last few weeks I've been exploring this sense of holding and releasing. I got back into doing the merging-with-the-jhana thing because it seemed to have features in common with the experience I'm looking for in my insight practice (exactly as you describe), but I *believe* the experiences of centrelessness that I'm now moving in and out of are a feature of my insight practice rather than concentration attainments. At this point in my sits I've run the jhanas 1-8, come out the other side into open awareness, and then started to poke around with my sense of holding on, witnessing etc. So it feels more insight-y to me.
I've seen plenty of people describe their 'untangling the knot of perception' as a kind of one-time event - boom, it untangles and stays that way. That's definitely not what's happening here - I'm sliding into a 'looser' way of experiencing, and then sliding out again. (It's been interesting watching what happens as I go back and forth through that process of more holding-less holding-more holding.)
The way I've been interpreting this, I'm seeing it as an insightful meditation experience which has not yet stabilised/matured into full realisation yet. My 'felt sense' of non-duality has been gradually deepening over the last few years without any particularly dramatic watersheds, and this feels like a continuation of that process. It lacks the nice clear-cut 'big bang' nature of some of the experiences I see being talked about, however. So part of me wonders if I'm doing it wrong, or this isn't a thing at all; and part of me wonders if it's just that sometimes people have dramatic shifts (which are easy to identify and refer back to) whereas sometimes people have a slower process of dawning realisation, and right now my experience fits into the latter category.
Perhaps your post suggests a third alternative: that what I've actually done is find a way to apply a concentration attainment to a state which is not obviously jhanaic as I understand it (this feels much more 'ordinary' whereas jhanas have never ceased to feel 'altered' on some level).
No mind experiences will be intermittent until a quantum shift of perception via realization
https://awakeningtoreality.blogspot.com/2018/11/no-mind-and-anatta-focusing-on-insight.html
https://awakeningtoreality.blogspot.com/2018/10/differentiating-i-am-one-mind-no-mind.html