If Yù has appeared in your reading, the book is naming a kind of momentum that organises itself almost without effort. The single yang line in the fourth place gathers the five yin lines around it. The image is thunder breaking out from the earth — sudden, audible, and felt by everyone in range.

Classical commentary distinguishes this enthusiasm carefully from mere excitement. The character carries the sense of preparedness, ease, accord. The hexagram appears when the moment is genuinely ready for a large move — when installing leaders and mobilising force become favoured by the configuration itself, not just permitted.

What the book counsels is the use of the moment for its proper scale. 作樂崇德 — make music, honour virtue. The musical metaphor is exact. Enthusiasm done well harmonises many voices around a single cadence; done badly, it just gets loud. The hexagram asks the reader to notice the difference, and to act for the harmony, not the volume.

Yù's failure mode is the mistaking of crowd warmth for collective truth. The momentum is real. The cause it is harnessed to may not be. The book is permissive about action in this hexagram, but the permission is conditional on the action being the right one. Catch the energy and ride it toward something the ancestors would recognise as worthy.