[dcb]

Ah, it is a flywheel and a variator.

When you want to slow down you put energy into spinning up the flywheel. You choose low gearing in the variator to slow down faster and speed up the flywheel (that rolling pin shaped thing) faster.

When you want to accelerate, then you take energy from the flywheel (thereby slowing it down) and putting it into the transmission, again through the variator.

Adds 55 lbs to the car, limited boost, controlled by a button on the steering wheel.

Flywheel weight/max rpm/alignment/distribution and Gyroscopic effect on vehicle handling, unspecified.