Ivan le fou

CONT: Crazy Ivan is a U.S. Navy term for a Soviet submarine manoeuvre, characterized by any number of sudden and sharp turns, used by submarine crews to ""look behind"" them. Because of the acoustic distortions and noise caused by propeller blades, it is nearly possible for conventional sonars to detect objects directly behind a submarine. Thus, with sudden turns, the area where the sonar is not effective shifts relative to the current heading of the submarine, causing previous gaps in sonar coverage to be revealed while masking known areas.
OBS: The ""crazy"" part of the name comes from the fact that these manoeuvre were very sudden and ""Ivan"" was a common nickname used to refer to the Russians. A standard tactic of pursuing submarines would be to closely follow the Soviet submarine hidden right in the sonar gap, causing the U.S. submarine to go undetected.