feathered stick, barb radical (no. 6)
亅 is a rare radical character representing a hooked or barbed stick, used primarily as a component in other kanji rather than as a standalone word.
亅 is a pictograph of a hooked stick or barb, with the single stroke representing the shaft and the hook at the end.
Picture a single stick with a sharp hook at the end, like a feathered stick used to catch something. The shape 亅 is that barb.
For ケツ, imagine a feathered stick with a barb so sharp it can cut: cut -> ケツ, and the hooked stick catches what it cuts.