What does "touch off" mean?

Cause to explode or fire; also, initiate, trigger. For example, The boys touched off a whole line of firecrackers, or These disclosures will touch off a public uproar. This idiom comes from early firearms, which were set off by putting a light to the touch-hole. Its figurative use dates from the late 1800s.

Depict very precisely, as in He touched off Teddy Roosevelt as well as it's ever been done. [Mid-1700s]