Hi,
If you look a these two diagrams.
This is with the magnetometer, that measures magnetic flux, or density of the magnetic field lines.
In its sensing state below, it is measuring 3 lines(Simplified for this example),caused by the earths mag field.
When a metal object, in this case metal vehicle, passes close the field lines distort through the truck, because it is metal.
The number of lines now being measured is 5. So the truck can be detected by looking for quick level changes in the background earths mag field
Hope it helps..
Tom.. 