How does the Audi A8 use its 'mild hybrid' technology for improved fuel efficiency?

I've heard that the Audi A8 uses a 'mild hybrid' system - what is this and how does it help with fuel efficiency?

The 'mild hybrid' system used in the Audi A8 is a 48-volt electrical system that works alongside the car's traditional combustion engine. This system helps improve fuel efficiency by allowing the car to coast with the engine off at speeds between 34 to 99 mph, and also by providing some additional power to the engine during acceleration.

The Audi A8's 'mild hybrid' technology uses a belt-driven starter/generator, along with a lithium-ion battery. This allows the car to decelerate using regenerative braking, which captures energy that would otherwise be lost, and stores it in the battery for later use. This not only improves fuel efficiency, but also reduces emissions.