It's actually surprising to me just how differently an engine can perform with different tuning. The ECU can control all sorts of parameters depending on the en

It's actually surprising to me just how differently an engine can perform with different tuning. The ECU can control all sorts of parameters depending on the engine. Things like fuel/air mix, ignition timing, valve timing, shutting down some cylinders, skipping ignition cycles, a lot more than you'd ever expect. You can tune an engine to sound like a braap braap muscle car or have it be nearly silent. You can burn all your fuel to get a few more horses or blow black smoke, or you can tune it to consume no fuel, emit nothing, and produce nearly no torque. Plenty of people tune their engines with aftermarket kit and/or software. There are even simple plug and play kits of extremely dubious quality any average Joe can install. What these defeat devices do is detect the testing equipment or conditions and tunes the engine in a way that meets requirements. Once the vehicle is out of testing, it reverts to the factory tuning which usually gives more power at the cost of illegally high emissions. That's really all it is, there's not too much magic involved. Just simple cheating.