Legge's translation of the first line reads that the first nine, undivided, shows its subject treading his accustomed path, and that if he goes forward there will be no error. No image of a tiger appears yet; the line describes conduct that has not yet come near anything dangerous at all.
The line sits at the base of Dui, the lower trigram of Lake, in the lowest and correctly yang seated place, directly beneath the whole ascending structure of Qian, Heaven, that gives the hexagram its name and its central image of treading on a tiger's tail. At this first position, though, the tiger is still four lines away. The subject walks a path already familiar, ordinary conduct undertaken without ambition to test itself against anything greater, and Legge's brief line offers no complication because none yet exists.
Read against Treading's Judgment, which promises progress and success from the very act of walking behind Heaven without being bitten, the first line is where that walking is easiest, since it has not yet drawn close enough to the tiger for caution to matter. Plain, accustomed conduct is itself sufficient here, and moving forward in it draws no error only because nothing threatening has yet entered the path.