A 'Nixie' is a type of sonar decoy used by surface combatants. It is a towed device which makes noises like a ship really loud in the hope of decoying passive sonar-guided torpedoes. This is best used in conjunction with a stealthing system such as Prairie/Masker, which uses a shield of bubbles to reflect the ship's noise (acoustic energy) back onto the hull, which is coated with anechoic tiles to absorb the noise. All in all, you want the submarine and/or torpedo in the area to think you're where you're not.