Six at the top is not addressed to the querent the way the other five lines are. It shows a total obscurity, and a subject who climbed to the height of the sky and whose course now runs into the earth. Drawn in a reading, it usually describes the situation rather than the person: something that has darkened everything around it has also reached the end of its own rise, and the line is a statement about how long that will last.
That makes it a difficult line to act on and an important one to read correctly. It counsels no resistance, names no moment of collapse, and gives no method for hastening one. What it supplies is perspective for the people described by the other five lines, whose choices about flight, speed, confrontation, exit and concealment are all being made under something the classic considers temporary in a way that its subject plainly does not.
When the line changes, the upper trigram Kun becomes Gen and the figure becomes Grace, King Wen 22, fire beneath a mountain and light thrown upward across a form. The transformation is a comment on how such things look while they last. What ascended to the top of the sky was ornament laid over an obscurity, and Grace is the classic's own study of adornment, of what it can honestly carry and of how much of a situation a reader should ever expect appearance to account for.