hinge, shears, scissors
鉸 centers on a metal tool that pivots at a joint: a hinge, or a pair of shears/scissors that work by crossing and pivoting.
鉸 combines 金 (metal) with 交 (crossing, exchanging), suggesting a metal object that works by crossing or pivoting, such as scissors or a hinge.
Metal 金 that crosses 交: picture the two blades of scissors crossing at a metal pivot, or a hinge letting two metal parts swing across each other.
For コウ, imagine a metal hinge that goes 'ko-ung' as it swings: the 'ko' sound cues コウ, and the crossing metal parts make the hinge work.