trigger

n. 1 movable device for releasing a spring or catch and so setting off a mechanism (esp. That of a gun). 2 event, occurrence, etc., that sets off a chain reaction. v. (often foll. By off) set (an action or process) in motion; precipitate.  quick on the trigger quick to respond. [tricker from dutch trekker from trekken pull]

n. A catch to hold the wheel of a carriage on a declivity.