I think that is more of a theory than a practice, unless they use capacitors or some other method to store it. Batteries can't accept all that charge unless maybe you were going down a mountain.
Once, before hybrids, they were working with a flywheel concept, where the flywheel ran in a vacuum at kitchen blender-like RPMs, It had a motor/generator attached through a gearbox and when you put on the "brakes" by reversing the drive motor/generators, it would rev up the flywheel.
Then when you accelerated, the flywheel would become a generator and provide acceleration. They would have you either plug it in overnight to keep the flywheel running, or have it on a timer so it would be ready to go to work in the morning.
The flywheel/gas engine hybrid would be a pretty good experiment too, except the flywheel motor/generator is a little above the pay grade of your average automotive mad-scientist.
Its the motor.