If Suí has appeared in your reading, the book is naming a moment when the wise action is to follow rather than to lead — not from passivity, but from discernment. Thunder is the lower trigram, Lake the upper. Movement below, joy above; energy in motion, and a receiver willing to be moved.

Classical commentary is careful about this hexagram. Following is auspicious only when what is being followed is itself worth following. The book pairs the four cardinal virtues — 元亨利貞 — with the explicit clearance 無咎. All four conditions must hold. The thing being followed must originate in something real, must let life pass through it, must fit its moment, must hold a straight line.

What the book counsels is the work of discrimination that has to come first. Suí is not the absence of judgment; it is judgment exercised early enough that one can then move with the chosen flow rather than against it. The evening image in the appended text is the daily training: notice what the day is doing, and go inside when it does.

Suí's failure mode is the surrender of agency dressed as flexibility. The book is alert to it. Following the wrong current is not following at all; it is being carried. The hexagram appears when the reader is considering letting go of a plan in favour of what is emerging. The question to bring is whether what is emerging deserves the deference.