it's "like" a spark plug. In a way, that it is directly IN the cyllynder. It's a high-pressure injector that squirts fuel right into the cyllinder, as opposed to the intake duct. By doing that, it cools down the air-charge in the cyllinder, and the ECU can time each one to fire early, or later, depending on conditions, like a cold start. Thus increasing fuel economy. Since it cools the charge, a higher compression ration can be used - increasing performance.
If you go to
www.cadillac.com and click on the "learn more about the all-new '08 CTS" it'll take you to a flash tour where you can click on "videos" and then "powertrain" or "engine" I forget which it is. Anyways it goes into a detailed explanation of what makes this engine so great, and into a detailed explanation of DIG...