the technology is immature, and the nasty electrical shock the BMW Sauber mechanic took didn't help with people's faith in it! Bosch is developing a KERS system that will likely see use in hybrid cars within a couple years.
The system is actually fairly simple. you've got the electric motor, the flywheel storage device, and a battery. Under energy regeneration, power is taken off the drivetrain to spin the flywheel. The flywheel spins in a vacuum suspended on magnetic bearings. Then when you hit a button, power is taken from the flywheel, goes to the battery (or supercapacitor), then runs the electric motor. You could just have the motor and battery, and run the electric motor backwards to charge, but flywheel energy storage has the advantage of a fast "charge" (no chemical reaction to induce as in a battery, you just need to spin up the flywheel to max rpm) and weighs less than an equivalent battery.
F1 rules limit the time to six seconds and the motor's power, but in an aftermarket application the power could be bumped up to 80kW or so. Hybrid technology used in a not-so-green application
