I think an intuitive has to live by Investing themselves, bleeding themselves out for some Thing. You hear this in all their talk - for both sides. The Ne types, like St. Exupéry, say you have to die for your rose - you have to barter your life for something Else (in his example, a Tree). The introverted intuitives say the same thing. Gandhi’s ideas are full of it - cling with your whole soul to The Idea. To your Belief. Live for it and through it.
So, if you think about it, both intuitive types barter their lives for something - they just differ in what this is.
For the Extraverted Intuitive, it is something External to the self - something Out There. A tree. A book. A work of art. It may be a person (e.g. the Man bartering his life for the Boy in Cormac McCarthy’s The Road), but it will be an external sense of bartering - to keep them alive, to keep that person In the World, so to speak. As McCarthy once said, Nothing worth doing is worth doing unless it almost drives you to suicide.
For the Introverted Intuitive, it is something Internal. An Idea - a Belief. A Value. (Depending on the type.) This could be a Theory (e.g. an INTJ), or one’s understanding of religious Faith (an INFJ). But it’s going to have to be Total - in order for the Introverted Intuitive to feel really completed.
I think, for people that have it as a secondary function, it’s slightly different. It adds a ‘drive’ to their dominant attitude. The INxPs use Ne to drive them to make their ideas and systems meaningfully relevant to the world. The ENxJs use Ni to drive their ideas, to give their plans for people or institutions a sense of real purpose and fire.
Of course, this often puts these two attitudes at odds. The Ne is selling themselves out, to the last drop of blood, for The World. For the beauty of things external to the self. The Ni is renouncing that world in favor of the Inner World. The Ne will see the Ni as blind and possibly destructive (if they destroy external beauty while following their vision), while the Ni will see the Ne as devoid of values and flighty - pretty much like the external World that they want to renounce.