The company describes the Extreme Hybrid drive train technology in these terms:
The Extreme Hybrid™ is the first practical drive train for a Plug-in Hybrid Vehicle. Its two-part energy storage system combines Lithium-ion batteries with ultracapacitors. This “hybrid within a hybrid” energy storage system exploits the strengths of lithium-ion batteries (light weight and high energy density) and ultracapacitors (small size and high power density). Batteries alone, have high energy density but they must be greatly oversized in today’s hybrid vehicles to avoid deep discharges. Battery-only hybrids also require a powerful internal combustion engine for hill climbing and acceleration.
Adding ultracapacitors with their high power density and high cycle life allows the Extreme Hybrid™ Plug-in to achieve top speeds and rapid acceleration in electric-only mode equal to a conventional hybrid. The Extreme Hybrid™ design allows for a smaller internal combustion engine while preserving high vehicle performance. For a typical daily commute of 40 miles round trip, an Extreme Hybrid™ vehicle does not use its internal combustion engine at all.
In an article for Salon, Joseph Romm points out that by themselves, hybrid electric vehicles won't contribute enormous greenhouse gas savings until cleaner electricity sources come on line:
The big greenhouse gas savings would come about as plug-ins enable a major transition toward clean electricity and away from petroleum-based fuel, which is getting dirtier every year, as unconventional oil, such as Canadian tar sands, becomes more popular.
Unlike petroleum, electricity is poised to get greener in the future, especially as we fight climate change. Indeed, once we have a national cap on carbon emissions, plug-ins will drive even faster growth of the diverse and growing numbers of carbon-free electricity sources.
As promising as Extreme Hybrid technology looks, the challenge is still daunting. Romm also explains that to prevent catastrophic climate change, the average U.S. car will have to have 80 to 90 percent lower carbon dioxide emissions by 2050 than today's vehicles.