Sometimes called "Translating Tendency" it is the sideward drift to the left when you do an engine cut in a hover.
The bank angle in a hover (3 degrees or so) is needed to fight the force the tail rotor is shoving against the aircraft. This bank angle is proper when there is torque on the main rotor (and therefore thrust on the tail rotor). When you chop throttle in a hover, you put a boot full of right pedal to keep the nose straight. This reduces the tail thrust to near nil, so you must take out the left bank angle. If you don't, you get the left sideward drift.