Originally Posted by FricFrac

Its likely inductive coupling (kinda like the "wireless" chargers for the phone with 40,000 fart applications.... what's the commercial name? Oh yea the iPhone). It probably induces a current in a winding or trace on the PCB in the Fob when you place it in the fobgina. That gives the Fob power so the RFID can be transmitted to the ECU with a dead Fob battery. Its not a simple switch - its an inductive power supply.