The third line drops the dragon from its imagery altogether and speaks instead of the superior man, active and vigilant all the day, and in the evening still careful and apprehensive, in Legge's rendering. The change of figure marks a change of stakes. This is the topmost line of the lower trigram, the last position before the hexagram crosses into its upper half, and Chinese commentary has long read that threshold as inherently precarious no matter which hexagram carries it.
Unlike the first two lines, which describe a dragon simply being what it is, the third line describes sustained effort. The superior man is not resting on inherited strength; he is working through daylight and remaining wary into night, because the position between the lower and upper trigrams belongs to neither fully and offers no settled ground. Vigilance here is not fear for its own sake but the appropriate response to standing at a hinge point in the hexagram's structure.
The line's judgment is that this watchfulness, though it costs the superior man rest, keeps him blameless. Nothing catastrophic threatens him, but nothing is guaranteed either, and the text is explicit that only continued care through both day and evening secures the passage. Read against the hexagram's overall movement from hidden dragon to dragon in the sky, this line is the labor that the ascent actually requires, not merely its reward.