A detonator is built into the bomb.
A blasting cap is used as a primer.
Figurative use for something that sets off a major event, change, or reaction. Often used in social, political, or personal contexts.
That incident became the trigger for the war.
His single remark sparked the reform.
起爆剤 is literally a detonating agent and is used metaphorically for a catalyst that spurs sudden change, similar to the figurative use of 導火線 but with less sense of a delay before the effect.
起爆剤 (literally 'detonator') is used figuratively for something that sets off a chain of events, much like 触媒 in its figurative sense of a person or event that triggers change.
起爆剤 (literally 'detonator') suggests a sudden, explosive catalyst for change, while 呼び水 has a gentler image of priming or coaxing out a bigger reaction.
Compound of 起爆 (きばく, 'detonation') + 剤 (ざい, 'agent, substance'). The figurative sense is a natural extension from the literal explosive initiator.