From the aircraft manual:
2.2 Flux detector unit
Each system has one detector located in the wing to avoid magnetic disturbances. The detector unit consists of two coils oriented 90_ apart which sense the horizontal components (Cos and Sin) of the magnetic field. The coil system is kept horizontal by a fluid damped pendulum.
The output representing the direction of the mag- netic field is transferred to the computer where it is compensated for magnetic errors of the detector. The compensated signal slaves the heading in- formation with 1_ per minute and the result is thus magnetic heading. Each computer has a compen- sator card for adjustment of the magnetic error, which is normally set at the aircraft compass swing (maintenance).
The system also contains a quick heading slave function, which by means of a push button, slaves the magnetic heading related to present flux detec- tor readout. There are two HDG SLAVE buttons, one on each side, located underneath the EHSI:s.