Hexagram 63 has a property the commentarial tradition makes a great deal of: it is the only figure in which every line occupies its correct position. In the system worked out in the Ten Wings, odd-numbered places are proper to unbroken lines and even-numbered places to broken ones. In this hexagram, and in no other, all six agree.
That system of correct and incorrect placement is not in the oldest layer. It is a commentarial apparatus, developed centuries after the Zhou Yi's core, and it is worth saying so because the observation about hexagram 63 is so often presented as a fact about the ancient text rather than about the tradition of reading it.
Within that tradition, though, the observation does real work. The hexagram is named Ji Ji, After Completion or Already Across, the river crossed, the task done, everything settled. And the judgement immediately undercuts it: success in small matters, perseverance furthers, and at the beginning good fortune, at the end disorder.
That is the hexagram's argument. A state of perfect order has nowhere to go except out of order, and the completion is therefore also the beginning of the decline. The Image counsels taking thought of misfortune and arming against it in advance, which is one of the book's most straightforwardly practical instructions.
The name is worth taking literally. Ji Ji is most plainly rendered as already across, the river behind you. That is not an abstraction about attainment; it is the ordinary language of a journey, and the hexagram's warning is the warning of somebody who has watched people relax on the far bank.