blindstitch
絎 centers on a specific sewing technique: a blindstitch, where stitches are hidden from view on one side of the fabric.
絎 combines 糸 (thread) with 行, which likely contributes a phonetic element. The exact historical development is uncertain, but the thread component clearly relates to sewing.
The left side 糸 is thread, and the right side 行 looks like a path. Picture thread traveling along a hidden path through fabric, creating a blindstitch that is invisible from the outside.
For コウ, imagine a coat being sewn with a blindstitch: coat -> コウ, and the thread hides inside the coat's lining.