The element has a heating wire inside it that is insulated from the element itself. If the insulation is compromised, then current will flow from the wire to the grounded element. I think you can take it from here...
Cosmic is correct.
Allow me to explain a little more technically...
a) the inside 'heating wire' is energized at 120V
b) the 'element' and any mounting apparatus is grounded
the GFI measures leakage current in milli amps between a + b




















































