[indent]I wonder if these are the features of some of the "formless jhanas" or if they're a separate effect of deep concentration.[/indent]
When thinking stops here, the mind may enter those mental states that are called "formless".
Leading up to the release of one's own thinking here, but not actually released of own self and thinking and feelings, there can be desires to control/hold this sukkha, a desire to desire more sukkha, a desire that it not end, a conceit that "it's just sukkha", a conceit that "I have sukkha" ~ all kinds of personal thoughts, which impede the actual concentration, impede
suffusive concentration. Furthermore, if one has sense of self remaining to evaluate the perceived jhana (though a "wow"ing self perspective can be toggled when this mental concentration is newly experienced) , this incomplete 'semi-jhana' will eventually be demoted by the experiencer, because they ultimately know whatever they experienced, it still contained their own controlling mind and identity.
When suttas recommend that each mental concentration (jhana) be stabilized before moving on, I feel the reason for this is because in each concentrated state
being suffused in that mental condition (jhana) is known when something "I" couldn't do occurs related to that concentration. The mind just does something that I couldn't imagine doing, even if someone told me exactly what would happen. In first jhana, for me, I know "suffusive first jhana" occurs when the mind seem to magnetize to its object (be it breath, mantra, kasina, etc), and this magnetized mind a) can only be set up (indirectly caused) by my own deliberate, gentle, repeated/habitual subduing of myself (letting thoughts arise and pass without keeping hold of them and gently returning to the object again and again and again, till the mind is satisfied not to leave the object) and b) the unified mind offers an experience of/consciousness of object-awareness that is distinct from awareness having any personal perspective. 2nd jhana has its suffusive hallmark, and third jhana - sukkha - can indeed launch one into what, in hindsight, may seem like something that eliminated what I would think of as normal physical and conscious boundaries.
Gah, words ;)
What's this training good for if the suffusive mental states occur? It begins the work of undermining self-centeredness and the problems of greed and ill-will. What's that good for? Well, understanding seems to broaden, and living and dying are close-by, so each are bringing attention into vividness. I still get lost in chocolate pretzels and subtle guarding of them though (this aside is to say, having experience of concentrated mental states does not make a guru or a perfected being, not me anyway. Life is way more interesting/simple/wow to me since practicing these last few years, but I still have hindrances, though the problems I bring myself ~ distinct from those non-pleasant things that just are a part of being alive ~ are clearer).
These are just my reflections from my own practice. Everyone may just sit/do and peaceably study/watch how own minds are moving.
