I believe it came about while taking a new look at Back to the Future, but it’s an idea that’s been festering for some time.
There were discussions some time ago about how each throughline’s storytelling can be completely disconnected from the storytelling of the other throughlines and the storyform will still hold up. For instance an OS about a group of nazis in 1940s Germany, an RS about a couple of cavemen in an ancient French cave, an MC about an astronaut on a mission to Mars in the 22nd century, and an IC about a comic book artist from the 1990s. All stories in different times and places, so the RS is by default not about the Mc and IC.
The idea is that the throughlines are perspectives within a mind. What looks like a particle from one perspective will look like a wave from another, and yet all perspectives are looking at the same thing. So while the Os looks at relationships between mental processes (or Characters) from the outside, the RS looks at them from inside. And since everything from the RS perspective will look like an inside view of the relationships between mental processes, you will see relationships in the RS whether looking at the MC and IC or any two other characters.
Another way to look at it is that the storytelling is an analogy for what’s going on in the Storymind. There’s nothing that limits you to one analogy, so you can use Joe and Jane as an analogy for MC and IC, but use John and Marsha for RS, even if these characters are all in the same story and interact with each other.
Furthermore, the Main Character Player is only an analogy for the MC perspective while active within the MC throughline. When this same player is active within the OS, they are not an active Main Character because they are in the OS which is not the MC throughline. Similarly, when the MC player is active in the RS, they are not really a Main Character as they are not in the MC throughline at that time. So even when the RS is between the MC and IC players, it’s still not between the MC and IC, because those are perspectives and not characters.
Hopefully that makes some sense.