The mechanism is simple and the elaboration on top of it is enormous. A line is either unbroken, associated with yang, or broken, associated with yin. Three lines make a trigram, and there are eight of them, conventionally glossed as heaven, earth, thunder, water, mountain, wind, fire and lake. Stack two trigrams and you have a hexagram of six lines; there are sixty-four, which exhausts the possibilities.
Each hexagram carries a name, a short judgement, and six line statements read from the bottom up. The language of the oldest layer is compressed and often opaque: omens, fragments of ritual, snatches of what appear to be old rhymes, verdicts such as auspicious or no blame, and images of crossing the great water or seeing the great man. Some line statements plainly record concrete historical or agricultural situations whose original reference is lost.
Around this core sit the Ten Wings, appendices that turn the manual into a philosophy. The Commentary on the Judgements and the Commentary on the Images explain each hexagram in ethical and cosmological terms. The Great Commentary, the most quoted of them, argues that the changes of the lines model the changes of the world, that heaven and earth generate the myriad things through the interaction of yin and yang, and that the sage who understands the pattern can act in accord with it. It is in the Wings, not the core, that the book acquires the metaphysics later readers came for.
A consultation yields not only a hexagram but often a second one, since lines identified as changing turn into their opposites. The reading lies in the movement between the two figures, which is what the title means.