The magnetometers detect anomalies in the earth's magnetic field caused by magnetic material in objects carried by individuals.
Most weapons contain ferrous materials that are magnetic.
This approach, because it will not alert on such innocuous objects as keys, significantly reduces the false alarm rate. Currently available commercial devices tend to detect all metal objects. The system can detect weapons with even a small steel content, such as in the Exacto knives used by hobbyists. The system has proven that it is capable of detecting items missed by other systems, such as razor blades.