Watt,
That is exactly as iIunderstand it , and always have. Previously they were a core balanced relay, where the incoming current had to balance the outgoing current. If any current found a way to circumvent this circuit, it unbalanced the circuit and a fault was sensed.
They can only then find a way out of the active-neutral circuit,if the electrons can find some other/alternative way home... can only be via an earth fault.
Neutral and ground bonded at the switchboard, and separately distributed thereafter. A 2 pin appliance sitting on a plastic tray cannot trip any remotely decent leakage detector... and no current can leak out of the circuit.
If you step up to the tray and grab the 2 pin appliance with any part live or leaking, then the core/relay will trip.
A three pin appliance can trip on it's own, as there is now an earth point in that frame of reference on the plastic tray. The track to ground then can be as small as spider web across the active to earth at any point, mixed with moisture and dust.... or you grab the active bits.....
It cannot trip without an earth return of some kind as I understand it.
............oztules