knock, strike, rap, tap, button
扣 centers on a sharp, percussive action: knocking, striking, rapping, or tapping. The button sense is a natural extension—pressing a button is like a quick tap.
扣 combines 扌 (hand) with 口 (mouth), but the historical development is uncertain. It may have originally depicted a hand striking something, with 口 possibly representing a target or sound.
A hand 扌 beside a mouth 口: imagine knocking on a door with your hand, and the sound comes out like a voice from a mouth.
For コウ, picture a hand knocking on a hollow door: the deep 'ko' sound echoes—ko -> コウ.