When introverts run into a situation that their Dominant Function can't handle, they need to adapt by negotiating, going along with the limitations of low-bandwidth social interaction, or otherwise doing something that does not involve total control or understanding of the situation. The Tertiary Function provides justification for maintaining an Introverted approach directly opposite to the worldly expansiveness that the person is ready for.
Tertiary Si (INxP): "I can't possibly go along with this, I don't have any reliable concepts or map to anchor myself with: it's all arbitrary and untrustworthy and meaningless. I'd be diving in without any orientation; I'd be tripped up or harmed from any random direction, and my efforts wouldn't be cumulative. I just won't budge. I'll build myself some barricades and wait for the storm to blow over." The Secondary Function (Ne) would say: "Look around, shake up the pot, see what new arises, and deal with it imaginatively; there must be better alternatives available than being stuck here." (My secondary Ne says something more in the lines of "Pay attention to your surroundings and do the thing that will have the most interesting results. Don't be dumb just because you don't have any experience.")
Tertiary Ni (ISxP): "I can't possibly go along with this, 'cuz it's all a lie. It's all a set-up by the Man. I'd just be serving his interests and not my own. I'm not gonna be suckered by all this self-serving bull. No way, man, I gotta go my own way." Or "Why should I imitate the cool people? Despite their shiny clothes and social connections, they are still as vulnerable as anyone else." The Secondary Function (Se) would say: "This thing's bigger than you. Better just go with the flow, do what you can, trust your instincts to deal with what's right here and now."
Tertiary Ti (IxFJ): "I can't possibly go along with this, because it makes no sense. It's filled with internal contradictions. It's crude and not true to the real principles of how this works. It's trying to shove an inappropriately a priori conceptual structure onto the reality. I will have to go by feel, and where it leads I can't know until I get there. I've got to trust this groove regardless of people's arbitrary expectations." The Secondary Function (Fe) would say: "You've reached the limits of what you can do alone. You can make others feel important by telling them your needs and asking for their help, and yielding some genuine control to them. See who you can find common cause with."
Tertiary Fi (IxTJ): "I can't possibly go along with this, because it would mar my soul. It's not 'me'. I am a good person, and in order to maintain my integrity, I need to steer clear of this. This is the responsibility of those other people: it emerges from their souls, not mine, so it's their problem." The Secondary Function (Te) would say: "Do something. Take responsibility even if no one gave it to you, and go after some tangible gain right now, within the limits of the situation and your current understanding. That will improve your position, after which you can re-assess and plot a new course."