In the cylinder there is a small magnet which rotates with the cylinder. When the cylinder is rotated, either with a proper key or forcibly with pliers the magnet moves to a position close to the hall sensor unit. This activates one of two sensors in the unit and that sensor closes a circuit which causes a low voltage to be seen back at the BCM. The voltage seen at the BCM must match the voltage value for that vehicle stored in BCM memory. If it does not match then security is triggered. If someone tries to spoof the magnetic field it is likely the second sensor will trip and cause the voltage seen at the BCM to be too low also triggering security.
Here is a look at the circuitry inside the hall effect sensor unit. The circled resistor is the one responsible for the voltage value seen by the BCM. The resistance varies and I don't know how many variations were produced but I have seen color markings on the case of these parts which I suspect identifies the particular resistance.
And here I am experimenting with one...
