If Dùn has appeared in your reading, the book is naming a season in which the wise move is to step back. Two yin lines have risen from below; the four yang lines above them are still in command, but the configuration is changing. The hexagram appears at the moment when continued advance would mean engaging on terms set by something that is now ascending against the reader's interests.
Classical commentary is precise about the difference between this hexagram and defeat. Dùn is not retreat as failure; it is retreat as strategy. The character carries the sense of withdrawing in good order, with what one needs to preserve intact. The hexagram appears when the question is not how to keep pushing, but how to pull back without loss of the essential.
What the book counsels is the dignified withdrawal that holds the substance even as it gives up the ground. 不惡而嚴 — not with hostility, but with dignity. Keep distance from what is no longer worth engaging. Do not perform the withdrawal as defiance; do not stage it as moral statement. Simply step back, and let the stepping back speak.
Dùn's failure mode is the refusal to retreat — the conviction that the season is still the previous one, that the configuration has not changed, that effort will still carry. The book is firm on this. There are seasons in which forward motion is generative and seasons in which it is corrosive. The hexagram appears when the reader is in the second kind and has not yet noticed.