trigger
鉷 centers on a trigger, the mechanism that sets something in motion. The given meaning is narrow, so the core idea is simply a mechanical trigger.
鉷 combines 金 (metal) with 共, which likely contributes the sound. The exact historical development is uncertain, but the character likely originated as a specific term for a metal trigger mechanism.
The left side 金 is metal, and the right side 共 looks like a hand pulling a lever. Picture a metal trigger being pulled to fire something.
For コウ, imagine a metal trigger that makes a loud 'koh' sound when pulled: koh -> コウ.