Each key have a radio frequency transponder embedded in the plastic head with a unique electronic identification code.
In less than one second, the transponder then returns a unique RF signal to the vehicle's computer, giving confirmation to start and continue to run.
If the on-board computer does not receive the correct identification code, certain components, such as the fuel pump and on some the starter, will remain disabled.
This helped ensure that only the owner of the vehicle can make duplicate keys.
Many 2000 and later Ford vehicles use the E-PATS system, which uses an encrypted transponder in the key.