The concept is to use a highly developed flywheel to capture, store, and reuse vehicle inertia normally lost as heat during braking. Think of it as a mechanical hybrid that’s lighter and simpler than traditional battery and motor systems. Until now it’s only been used in racing, but Volvo has been hard at work adapting the idea to work in passenger cars.

Everything in the Volvo KERS system is built around the flywheel, but it’s not just a lump of round metal. It’s a wound carbon fiber shell over a lightweight aluminum core totaling 13.2 lbs and precisely balanced. This construction is optimized to prevent the structure from stretching outward due to centripetal force—important because this thing is designed to spin up to an unfathomable 60,000 rpm.