they always had regen braking. this is different: " Volvo’s KERS setup is based around a carbon fibre flywheel that spins at up to 60,000 rpm. Braking spins the flywheel up, and accelerating takes advantage of its momentum by supplying drive to the rear wheels."
Why would they make the flywheel out of carbon fiber? Isn't carbon fiber supposed to be very lightweight? In a flywheel you would want a massive wheel.
my guess that it's easier to spin-up during braking and it's less weight to carry during driving. I guess they went through a bunch of materials before they figure out price/performance/efficiency sweet point. Flywheel even enclosed in vacuum to minimize friction...