Instincts have certain "roles" and "applications" that are fixed in accordance to their positioning in the stacking. Even if one instinct is accentuated and expressed to an excess, it's "role" or "function" never changes because its position is fixed (same as in Socionics accentuated creative function never switches to become leading one no matter how strongly it is expressed). This is easier to see if you break down the instinct stackings into two parts, similar to model A. There are: 1) positions in the stacking (similar to functions of Model A), and 2) instincts that fill out those positions (similar to information elements). The combination of the two gives the full stacking. And while there's local accentuation, the position of the instinct in the stacking and the corresponding "role" of that position that the instinct will play for you never change.
Strongly expressed Sp was the reason I typed as Sp/Sx for several years when I was first introduced to the enneagram. Because most of what I could see of my life was Sp Sp Sp and more Sp. It took a while and some introspection to realize that the Sp part of me was always turned outwards, to the outside, towards the environment, that this was how I dealt with my surroundings. When I looked inside deep within, it turned out it wasn't the primary concern, and in fact it would get overriden and flung aside. There was a pocket of feelings hiding there instead.
The 'syn' and 'contra'-flows provided an additional clue - I kept attracting So/Sx's to myself as friends, which was described by at least two people on the old Enneagram Institute board, both of whom typed as Sx/Sp. This feedback was later incorporated by David Gray as part of his
syn/contra flow theory.
So putting these clues together it 'clicked' for me at some point that my real stacking was Sx/Sp and that due to heavy accentuation of secondary instinct I have mistaken it for my primary one. I've developed some prejudice against Sx/Sp stacking initially, as someone who hasn't grown out of their highschool angst, so realizing that was my own stacking it was like "Noooooooooo!" LOL
Nowadays that I've got this cleared up, I see other people make this same mistake all the time: mistaking strong secondary for primary. Especially newcomers to the instinct theory, since this isn't properly explained and expanded in any instinct sources that I've seen.