They disable Lojack systems, too. All they do is shear off the transmitting antenna.

I went to an electronics store and bought a small printed circuit socket (like the diagnostic ports) and printed circuit board with contacts, and spliced it into the ignition wire. No crank, no spark unless the little circuit board was plugged in. I drilled a hole in it to hang on one of the two part keychains, mounted it under the edge of the underdash plastic, and just slipped the board in to activate the circuit. Anyone with half an hour, a flashlight, a jumper wire and a multimeter could have defeated it...but this is not your typical car theft scenario. It was especially useful since this car had a punched out ignition and would otherwise start with the screwdriver I kept on the seat. It would have been even more effective with a noisemaking alarm system.

A really cool solution using new technology would be to get one of the magnetic solenoid, where you pass a small ceramic magnet past a hidden relay to magnetically activate it, then start the car. Once the ignition is shut off, the relay deactivates and the ignition is disabled.