If you're curious how the much-heralded two-mode hybrid system being developed by GM, DaimlerChrysler and BMW works, check out this explanation at AutoSpeed. Co-written by engineers from all three automakers, the tech feature not only goes into the mechanics of the hybrid systems but also the economic factors that drove its development. I saw this system for the first time about 18 months ago at a GM Powertrain preview and was very impressed with the product's versatility. At that time only the large version, which will be used in SUVs, was shown. But now I can see a smaller model that will used in luxury cars. Some may wonder why a version for compact cars doesn't have early priority. GM has long professed addressing vehicles with the worst fuel economy. That's why the two-mode strategy was first tried out in bus fleets. I don't know the exact numbers, but a single hybrid bus probably saves more fuel each year than a few dozen Prius models. Also, premium and higher-priced vehicles can help offset early development costs. Cynics will scoff at a 25 percent increase in fuel economy for a vehicle that only gets 15mpg to start with, but in the long run the two-mode can make a difference.