The discovered attack is unusually elegant and powerful form of double attack. What happens in this case is that a unit moves off a line in order to "discover" (actually uncover) an attack by one of its colleagues. What usually happens is that the discovering piece simultaneously unleashes a secondary threat of its own. Very often it is impossible to parry both the threats. The inexorable beauty of such situations is that the defender has all sorts of resources, but each one fails. Discovered attacks are particularly nasty when they arise froma plausible series of preliminary moves.