"Compass swinging" is a procedure for improving the accuracy of a magnetic compass employed in a vehicle (car, boat, aircraft) whose magnetic field (due to ferromagnetic materials or electric currents) creates a local distortion of the earth's magnetic field. The procedure involves "swinging" the vehicle - turning it to multiple directions - while adjusting "compensators" built into the compass. Typically the compensators are small magnets that can be turned through screws accessible while the compass is in its operating position.
An atypical but interesting "swinging" procedure was used during the preparation for launch of the WW II German V-1 missile. Since the missile intended path was in a straight line from the launch point to the target, the need for compass accuracy was only for that line. The missile was therefore suspended in the appropriate direction in a non-magnetic building, and the warhead's steel casing was beaten with wooden mallets until its magnetic field was properly aligned[1].