First of all according to MBTI INTP doesent use Ni as any sort of function, some MBTI spin offs do say that tho(like beebes typology for example), as does socionics.
According to jungian typology(from which MBTI, socionics etc were derived from) and MBTI there are 4 functions, T, F, N and S, introversion and extraversion of these functions are habitual ways of using those functions. In other words Ni and Ne are not 2 functions, but the function is N, and Ni and Ne are different habitual ways of using intuition. Ni users have a habitual way of using intuition as a means to abstract(remove irrelevant data from X to get to the underlying cause/answer/reason/whateverinquestionatthetime), while Ne users habitually use intuition to add and relate to something external and that way "see behind the scenes". INTPs have an abstract attitude towards thinking, which means that an INTP would first try to see all the possibilities using intuition and then remove what doesent make sense from thinking point of view.
I would suggest clicking the "read" text on my signature and start reading.