Trigger

actionneur

(similar device used to activate any mechanism)

activateur

(event that initiates others, or incites a response)

détente

(finger-operated lever used to fire a gun)

gâchette

(finger-operated lever used to fire a gun)

déclencher

(to initiate something)

être évocateur ; déchaîner

(to spark a response, especially a negative emotional response, in (someone))