As a changing line, nine at the beginning of Treading reads as permission to proceed along a course already familiar, without yet needing the caution the hexagram will demand later. A querent drawn here is shown a path plain enough that going forward causes no error, since whatever danger the hexagram's central image, treading on a tiger's tail, eventually names has not yet been approached at this earliest position.
Flipping this line changes the lower trigram from Dui to Kan, Water, giving Qian over Kan, the hexagram called Conflict. The ease of an accustomed path, walked without incident, gives way on the far side of change to a hexagram built entirely from contention and dispute, heaven and water moving in opposing directions rather than one following safely behind the other. The movement is from untroubled ordinary conduct toward open disagreement, a reminder that the same plain confidence that draws no error here can, if the position shifts, become the friction Conflict describes.
Within Treading's own arc, the first line is the point furthest from the tiger, before the hexagram's real subject, cautious conduct in the presence of overwhelming strength, has become relevant at all. A reading landing here suggests the present course is safe to continue exactly as it is, with the caution the later lines will require not yet due.