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A knuckle is a defensive technique used by a submarine under attack. A submarine makes a knuckle by a sudden, sharp movement back and forth of the rudder. This leaves an especially tubulent spot in the water behind the submarine, and then the sub continues moving away. Because of how sound waves are bent and reflected in water, this "knuckle" can confuse an inbound enemy torpedo's target-homing sonar, and sometimes even fool the torpedo into thinking the knuckle is the actual submarine's hull. The result is thus hopefully that the torpedo (or the enemy weapons techs controlling the torpedo thru its guidance wire) will lose contact on the submarine, or else the torpedo will actually detonate against the knuckle, with little or no harm to the escaping sub itself. Knuckles can also be used to confuse/defeat active sonar being used by surface ships, sonobuoys, ASW helicopters with dipping sonars, etc.