Double Knock : A technical term relating to a warning or fire extinguishing system (e.g. sprinklers) that will not activate until two physically separated detectors have been triggered
Business Continuity Institute, Lyndon Bird (FBCI), Dictionary of Business Continuity Management Terms (Version 2), Caversham, UK 2011. http://www.thebci.org/glossary.pdf, (BzCntMgt - Brit) {18}