The fifth line is the hexagram at its best. Legge's translation calls it the admirable retiring and attaches good fortune to firm correctness, while the commentary on the image locates the whole merit in the rectitude of the purpose behind it. There is no imagery at all: no tail, no thong of hide, no binders. The line is a bare statement that a withdrawal can be excellent, and it is the only place in the figure that says so without any qualification.
Structure supports the claim. This stroke is strong, central and correctly placed at the ruling seat of the upper trigram Qian, the only line in the hexagram to satisfy all three conditions at once, and it stands in proper correspondence with the yielding second line below. Retreat conducted from that position is neither flight nor stubbornness. The second line held a purpose that could not be broken, and the fifth is what such a purpose looks like when it also carries the authority to act on itself.
What the text does not supply is a rule for when. The judgement of the hexagram grants progress and success, and allows that firm correctness is advantageous to a small extent, so the whole figure is calibrated to modest and well judged action rather than to decisive movement. Excellence here consists in a matter of timing that the classic will not reduce to a formula, and the appeal to rectitude of purpose is as close as the commentary comes to naming what does the judging.