The second line says almost nothing, and the silence is its content. Legge's translation gives only that with firm correctness there will be good fortune, and the commentary on the image supplies the whole explanation in one clause about the subject being in the centre. No toes, no ram, no fence. In a hexagram crowded with animals and barriers this is the one place where the text declines to picture anything at all.
The reason lies in the structure. This stroke is strong and central in the lower trigram Qian, the seat from which the lower half of the figure is governed, and it stands in proper correspondence with the yielding fifth line above. Centrality is the classic's standing name for strength that needs no image because it is not doing anything visible with itself. The lines around it are all cases of force meeting something: toes advancing, horns catching, a fence opening. This one is force at rest in the right place.
Read across the whole hexagram, the line functions as the standard the others fall short of. Great Power is a figure about excess, and its judgement offers only that firm correctness will be advantageous, withholding the usual promises of progress and success. The second line is what that judgement looks like when a single line satisfies it completely, which is why it needs no picture and receives no qualification.