The sensing coil
The ring is a toroidal current transformer. Hot and neutral both pass through its centre, carrying equal and opposite current in normal use. The net flux is zero and the winding on the ring produces no signal. A fault — current finding earth through a wet hand, a cracked tool housing, a puddle — unbalances the two by exactly the fault current, and the winding produces a voltage proportional to it.
The window: 4 to 6 mA
UL 943 Class A devices must trip when the imbalance reaches 6 mA and must not trip below 4 mA. The number is not arbitrary: the research behind the standard put the threshold of ventricular fibrillation for an adult well above it, and ordinary leakage in a house — a few hundred microamps from a motor or a heating element — well below it. A GFCI that tripped at 1 mA would be useless; one that tripped at 30 mA is a fire and property device, which is what a European RCD is (see GFCI vs RCD).
The clock: how fast
The standard gives a curve, not a single time: the larger the fault, the faster the trip. T = (20 ÷ I)1.43 seconds, with I in milliamps — about 5.6 s at 6 mA, 25 ms at 250 mA. The electronics compare the coil signal to the threshold, fire a solenoid, and a mechanical latch opens both conductors.
Why it needs no ground wire
Nothing in that description used the grounding conductor. The device compares hot against neutral; the fault current can return through the earth, through plumbing, through a person standing on a wet floor — any path but the neutral. That is why the Code lets a GFCI protect a two-wire circuit in an old house (older homes), and why a plug-in tester with a ground pin cannot test one on an ungrounded outlet (testing).
The two buttons
TEST puts a resistor across hot and the far side of the coil, creating a deliberate imbalance of about 7 mA; if the device trips, the sensing chain works end to end. RESET closes the latch. Since 2015 receptacle GFCIs also run that test on their own (self-test devices).