The third line is where the darkening is struck at its head. Legge's translation places the subject hunting in the south, taking the great chief of the darkness, and then warns in the same breath that he should not be eager to set everything right at once. The two halves pull against each other on purpose. The decisive act succeeds, and the classic spends the remainder of the line telling the successful party to slow down.
The south belongs to fire and to brightness in the standard correspondence, and this stroke is the topmost line of the lower trigram Li, the strongest and most exposed point the light occupies before the earth begins. It is yang in an odd place and therefore correctly placed, and it stands opposite the topmost line of the figure, where the classic puts the dark ruler. The confrontation is structural rather than incidental: this is the only line that faces the source of the darkening across the whole hexagram.
What the warning adds is the difference between removing a person and removing a condition. The great chief is taken and the darkness is not thereby over, because what produced him is a settled arrangement, and settled arrangements outlast the people at the top of them. The classic does not withdraw the victory. It declines to let a reader treat the victory as the end of the work, which is the same restraint the judgement of the hexagram asks for when it says the difficulty of the position must be realised.