How exactly does the coolant level sensor work?

I notice (on its inside that goes into radiator) it has a small copper like pin coming out of it in an open round plastic piece and has two connections inside its plug.
It looks like the coolant can only make contact with the copper pin, so how does the circuit complete itself to tell you the coolant level is OK or too low? In other words, coolant above the pin or below it.

Anybody know exactly what is inside this sensor, any diagram you know of posted somewhere?

It is GM part number 10096163

Al