dead man’s switch
n.— «This pyrotechnic crew won’t be sitting around, though, until the “dead man’s switch,” as it is called, is clicked to start the show. Until then, they’re moving boxes, cutting tape and sorting through thousands of fireworks.» —“Behind-the-Scenes with Pyrotechnics” by Britney Glaser KPLC-TV (Lake Charles, Lousiana) July 4, 2007. (source: Double-Tongued Dictionary)