Background on the Stuxnet worm / by Eric Dacus

One part of the program is designed to lie dormant for long periods, then speed up the machines so that the spinning rotors in the centrifuges wobble and then destroy themselves. Another part, called a “man in the middle” in the computer world, sends out those false sensor signals to make the system believe everything is running smoothly. That prevents a safety system from kicking in, which would shut down the plant before it could self-destruct.

via the New York Times