Unrelated to the latest posts (but related to the topic in general), I read some on Jungian VI and found an interesting excerpt:
Essentially, in highly introverted individuals the dominant function may not manifest externally, and instead cruder extraverted functions surface.When the introversion is pronounced, and this is my personal observation which may depart from Jungian tradition, it may be that both of the superior functions are introverted. In such a case, the auxiliary still comes to the surface but it is the strongest of the visible functions. Its introverted nature can be detected by a sense that the attention is focused on ideas rather than on what is directly in front of the individual. The fact that the auxiliary is pulling energy inwards prevents the dominant function from surfacing at all: the other side must manifest extraversion, and because the auxiliary, which is holding the fort, so to speak, must work in tandem with a function of the opposite category and polarity, the other function to surface will forcibly be the inferior function. At least, this gives us a clue as to what is happening, for the inferior function has what Jung referred to as a "primitive" quality. When the reconstruction of the facial halves reveals one side that seems shockingly foreign to its owner, determining this alien side's function can help us find the dominant side, for it will be the polar opposite. If awkward feeling is displayed, then introverted thinking might be the dominant function; if one of the perceiving functions appears in a primitive form, then the other perceiving function might be operating as the superior function, although hidden in the background.