Jung describes introverted sensing like this, in Psychological Types:
I guess I would imagine an Si dom to be someone who can navigate the 'mirror world' with ease, and utilize it for various efforts. Whereas someone who didn't have Si dominant might still find themselves in such a place, but would experience it much more clumsily and it may appear more magical or something than if it were to be one's home.
"Introverted sensation apprehends the background of the physical world rather than its surface.
The decisive thing is not the reality of the object, but the reality of the subjective factor, of the primordial images which, in their totality, constitute a psychic mirror-world.
It is a mirror with the peculiar faculty of reflecting the existing contents of consciousness not in their known and customary form but, as it were, sub specie aeternitatis, somewhat as a million-year-old consciousness might see them.
Such a consciousness would see the becoming and passing away of things simultaneously with their momentary existence in the present, and not only that, it would also see what was before their becoming and will be after their passing hence.
Naturally this is only a figure of speech, but one that I needed in order to illustrate in some way the peculiar nature of introverted sensation. We could say that introverted sensastion transmits an image which does not so much reproduce the object as spread over it the patina of age-old subjective experience and the shimmer of events still unborn.
The bare sense impression develops in depth, reaching into the past and future, while extraverted sensation seizes on the momentary existence of things open to the light of day."