Most of us have probably had the experience of watching a movie or TV show, or reading a book, and forgetting that we are doing so. We become absorbed into the fiction, and the outside world drops away. You no longer sense the person sitting next to you in the theater. You no longer notice you are turning pages. That the real world exists is forgotten for the moment. That, in a nutshell, is immersion. Immersion is sought in RPGs for the same reason it is sought in movies, TV, theater, and books.
This is a good description, similar to what I was trying to elucidate in the OP. And I'd say that most of us have had an experience like that, at some point, in our RPG play "careers."
But in my experience, it's tenuous and fleeting. The very few times it happened, I remember thinking that it was an interesting, enjoyable feeling, but I couldn't pinpoint what it was that would have led to that moment.
Interestingly, the strongest, most vivid "immersive" experience I had was playing a system I strongly dislike (GURPS). And if I can remember it correctly, it seemed to be strongly tied to a scene where the GM and I mostly ignored the rules, or at least the rules weren't a factor in the play out of the scene.
Now, in RPGs, we are usually talking about a subset of that kind of immersion - immersion in the first-person experience of a single character. But the reason that experience is important to some is still the same.
And this is where it starts to get interesting. Because after my recent experiences with Ironsworn, I'm no longer convinced that first person / single character perspective is a requirement for "immersion." Two sessions ago there was a dramatic moment where I can't recall being as "immersed" / in the moment / in suspense as a GM, ever. I was completely enthralled with the drama playing out in front of me, and I had almost zero final authorial say in the outcome. I wasn't "immersed" in the view of a single character, I was immersed in the scene as a whole.
Mind you, I think the assertion that you cannot be role playing while using dissociative/non-diagetic mechanics is like saying an actor cannot actually be feeling emotions while remembering their blocking, where the camera/audience is and adjusting to suit, or recalling their lines. I can accept that maintaining immersion in the role can be harder when your mechanics are outside the fiction, but it is not impossible. Humans aren't great at multitasking, but we can sometimes achieve it to some degree.
And the whole "association" thing goes back to the idea that "immersion" is best realized when you can only "see the world" / interact with the world / interpret the world as your character. I think this is much of what I'm trying to get at with this thread --- how, when, and why did this particular conceit become accepted as
the mode of play that leads to that fleeting sense of immersion?
Because without that particular conceit, the entire dissociated mechanics argument falls apart. If immersion can take place outside and beyond the confines of "playing only through my character," then the need to assert the primacy of "associative" mechanics falls apart.
In retrospect, though the concept of "associative" mechanics is tautologically correct ("decision points taken by the player correlate to decision points made by the character), as I review the last decade of my RPG play, I don't ever really recall my group having "immersion" as a goal of play. Enagement? Absolutely. Of paramount importance. The action, the dice, the drama / stakes of a particular scene, the hijinx and laughs . . . all of that.
But immersion? Hmmmm . . . not so much. And I'm sure there's some D&D purist that would claim that 8 years of playing Savage Worlds has blinded me to SW's "dissociations," such that of course immersion was impossible! "Don't you know how dissociative Savage Worlds is? Of course there was no immersion!"
But if the goal is to really experience that "slipping away of the real world into the imagined," how does mechanical "association" and "only acting through my character" really accomplish that?
Because when I look at the strongest time I was "immersed" while playing GURPS, it seemed to have more to do with the GM allowing me to
advocate for my character and
accept my improv'd propositions as truth.
Now, I will say, for that particular game, I had probably the most strongly-realized character I've ever played. For all of its fault in resolution, one of GURPS' great strengths is how very strongly realized its characters are. The granularity and minute detail that go into a character build create very strong "building blocks" for knowing who your character is. My immersion was aided by the fact that I could easily slip in and out of "actor" stance without losing the thread of my character's motivations. I could quickly survey a scene in "author" stance, and know how my character in abstract would respond.
So I don't think "immersion" is achieved just through "playing through the eyes of the character." Playing through the eyes of a weakly-realized, rootless-vagabond-murderhobo isn't immersive in any way.
And thinking it through now, the immersion achieved in Ironsworn was greatly faciliated in a similar way---all of the characters are strongly realized through simple, but effective "fictional positioning" effects.