They use a magnetic tip and a strain gauge. The magnet sits off the surface slightly and instead of a spring it is attached to a strain gauge, the harder the magnet pulls the thinner the paint. The strain gauge send signals to the processor for the reading. They work using something simular to a LVDT, http://www.macrosensors.com/lvdt_macro_sensors/lvdt_tutorial/lvdt_primer.pdf I have no idea how the electronic gauges work for non-metallic panels!
