Maybe the most devastating tactic in chess is the double check. I once heard it referenced as the atomic bomb of chess! Double check is the simultaneous check from two chessmen. The check(s) cannot be blocked; thus the only escape is to move the King. It occurs when a piece moves checking the King, uncovering another piece which also checks the King. In other words it consists of a check and a discovered check. For your viewing pleasure, I included a png of an internet game I played. Not stellar chess by any means but it certainly demonstrates the power of the double check, even leading to checkmate. Enjoy!