There are two electric motors inside the automatic transmission which react to driver input and the perceived needs of the gasoline engine (adds more torque for hauling or going up a hill, less for downhill or cruising on city streets, etc). Combined with a 6 Liter V8 with the "Active Fuel Managament" system that can run the engine on all 8 cylinders or shut down 4 of them to also save fuel, GM says they have cut the fuel consumption of their Tahoe, Silverado and other big trucks and SUVs by up to 50% under certain circumstances. For this, GM was awarded the "Green Car of the Year" prize from industry publication Green Car Magazine. The transmission was developed by GM, Mercedes, Chrysler and BMW over the past three years in Europe. It seems to be a big step forward, but the basis of the system is still the internal combustion engine; there's no new battery technology, no new alternative fuels being developed and used, and those are just two starting points where car companies might plan to spend a lot of time and money in the near future.