Perhaps what Lenore means by Introverted Perception is an attitude of putting things you experience into categories, where the categories are chosen on the basis of whatever seems to you to fit the things, without serving a predefined purpose or criterion.
Taking an Si perspective, then, you simply find categories to put things in, and use these categories to build a rich network of mental associations that guide you to attend to the things that matter to you--to find those things in the midst of a predominantly overwhelming perceptual field.
Taking an Ni perspective, you attend to the nature of whatever categories you come across: what they contain and what they leave out, what they assume about the context where they're applied, what (probably unstated) purposes are served by those categories, what cannot be said in terms of those categories (and that, if said, might unravel their power to seem real and meaningful).
Si leads you to gradually accumulate a factual map of the world, or at least the parts of it that are of interest to you. Ni leads you to gradually accumulate an understanding of how different maps operate and to be able to compare the assumptions of different maps against each other.