Flywheels are known to be very good mechanical energy storage devices. They have a certain weight and, by spinning, they capture the energy in their momentum. That energy can be released by electromagnetic means and transformed into electricity.
Although there are some safety issues associated with using flywheels in mobile applications, as Chrysler’s experience proves it, there is a company, Torotrak, that uses flywheels to capture the energy lost by braking, and reuse it while the car accelerates. For a few seconds, the flywheel is able to output about 80 horsepower and boost the car, thus saving fuel.
Enough with the words, the video is self-explanatory: