Williams was the last hold-out in F1 that was considering running KERS in 2010. The system developed by WHP was unique in that it combined a mechanical flywheel for energy storage with an electric motor/generator. The generator would spin up the flywheel during braking and the flywheel would turn the motor under acceleration.
WHP is hoping that some automakers will adopt the system as an alternative mild hybrid system that doesn't require a battery. Jaguar/Land Rover are involved a research program using a purely mechanical flywheel hybrid but may also be looking at the Williams approach.
[Source: Racecar Engineering]