Injector cut. When I did mine on my 2.0L ZETEC, I also tried the cam sensor and the crank sensor, and neither one would kill the engine. I did an ignition cut for a while, but the unburnt fuel hitting the cat caused a SES light. I then did a normally closed momentary switch in the common feed wire to the injectors and it has worked great for more than a year. Got to get pics of my "stealth" kill switch up someday. Put the switch in the center console so the release pawl on the bottom of the e-brake lever contacts the microswitch. To kill the engine, I just push the button on the end of the e-brake.
And no, it will only work when the brake lever is all the way down, the engine does not die when releasing the brake if it is engaged.