The second line is where the retreat acquires something to hold on to. Legge's translation has the subject holding his purpose as fast as if bound by a thong made from the hide of a yellow ox, and states that the thong cannot be broken; the commentary on the image glosses the picture as firmness of determination. Nothing in the line moves. What it describes is an attachment strong enough that no external pressure can loosen it, at the exact moment when the hexagram is otherwise about letting go.
Both halves of the image are chosen rather than decorative. Yellow is the colour of the centre in the Chinese scheme of five colours, and this stroke holds the central place of the lower trigram Gen. The ox is a yielding beast of burden and not a fierce one, and the line is itself a yielding stroke, correctly placed in an even seat. So the firmness on offer is not that of a strong line forcing an outcome. It is the toughness of hide, which holds because it gives.
Set against the first line, which had nothing to grip at all, this one has found the thing the retreat is for. A withdrawal that abandons its purpose along with its ground is a rout rather than a retreat. The hexagram approves of retiring, and this line specifies what must not retire alongside it, which is why the whole weight of the image falls on a binding that cannot be broken rather than on any distance covered.