The theory of Will to Power" that we find in the Nachlass is a highly complex world picture described from many angles (KGW VIII 3, 46). First off, Nietzsche does not intend the term Wille to refer to mere human "willing" (KGW VII 3, 334-336; VIII 3, 186), but rather he refers to the will to power as a vast, interlocking struggle of indefinite "power quanta" preserving and enhancing constantly changing power constellations (Machtconstellationen) (KGW VII 3, 338; VIII 3, 162-3; VIII 2, 55). The image here is one of ontological strife in which power quanta (Machtquantum) or power centers (Kraftcentrum) unfold in a "lawless" Becoming whose only underlying "reality" is preservation and enhancement of power (KGW VIII 3, 49-52, 163; VIII 2, 278). Hence, Reality reduces to pure Becoming.
This structure of will to power penetrates all modes of existence. It is the underlying ground of both the inorganic and organic (higher and lower) realms (KGW VIII 3, 53-4; VII 3, 235). As we approach the species of Human Being the organizational alignments of conflicting power centers achieve a higher level of complexity. But note well, this complexity is only a matter of degree, not kind–the human species is merely a "specific instance" of the general principle of Will to Power. At this highest level of complexity we find "consciousness" (the Ego) arising out of a necessary requirement to simplify and order the aggregate of conflicting power struggles which constitute the chaos that is man. Thus consciousness itself is merely an epiphenomena of will to power (KGW VIII 3, 93; VIII 3, 382, 325; VII 2, 289). It functions with the same tendency that drives all Becoming, viz., the preservation and enhancement of power. And it is in the light of these criteria that we find, within "the theory of Will to Power," the evolution of knowledge and truth (KGW VIII 1, 262-298).
It is through consciousness and "its" Will to Power that the human species schematizes the vast chaos of changing power configurations and imposes an order upon Becoming (KGW VIII 2, 46). This ordering of that which is in principle "beyond order" involves the necessary simplifying (and thus, for Nietzsche, falsifying) of a changing manifold. But we must be careful here, for what occurs with consciousness is extremely subtle. It is not as if we impose certain categories upon changing appearances (al la Kant). Rather, our "consciousness" is merely a perspectival point that seeks to gain control over that which surrounds it, and in so doing it creatively constructs a world. A systematic perspective becomes the world -- there "is" nothing beyond it. Thus the world ("Reality") is the perspectival interpretation we have of it (KGW VIII 1, 323, 259).
This is Nietzsche's "theory of Will to Power." It encompasses the whole sphere of existence, from the inorganic to the amoeba to man. It seeks to give an account of consciousness, knowledge and truth in terms of the preservation and enhancement conditions of power centers. In discussing these structures of human existence Nietzsche adds the problem of value and describes the "movement" (decline or increase) of Will to Power in terms of valuation. The ultimate criterion for human power involves the manner in which it values life, i.e., "Does this will serve to strengthen itself or weaken itself?" This at once affirms that the source of all (human) "forms of life" is will to power and also that some forms seem to enhance this drive (e.g., the artist) while others seems to negate this drive (e.g., the Christian Moral Ideal). It was this insight that lay behind Nietzsche's whole critique of Christianity and his "theory of Will to Power" served to buttress this position from the inside by providing a worldview which could account for the opposition of worldviews (e.g., "Dionysus vs. the Crucified") and establish within itself a criterion for evaluating worldviews without any appeal to "outside truth."
Now it is exactly at this point, when "the theory of Will to Power" entails the problem of value, that we can catch a glimpse of the "theory behind the theory." At bottom, Nietzsche seems to be recommending a way of looking at things (i.e., a perspective on things) that, if we were to adopt it, might lead to a more enhancing way of looking at the world (of Being-in-the-world). He seems to be proposing a certain kind of "lie" that could serve to enhance life. In this sense, his "theory of Will to Power" would seem to be a function of Will to Power itself. In fact, one can speak of "the notion of Will to Power" as underlying "the theory of Will to Power."