The best way to explain the benefits of the Audi system is by first establishing an issue with conventional start and stop systems. With a conventional system, there is a palpable moment between when the foot comes off the brake pedal, and when the engine starts. This moment is long enough that it could result in, at minimum, annoyance if the engine were shut off while slowing down, but traffic started moving again, and you couldn't accelerate immediately. At worst, this could result in a somewhat hazardous moment when you don't have full control of the car's movement.
The belt-driven alternator and starter on the Audi system can start the engine much faster, and it has sensors that can monitor the car ahead to know whether the engine needs to restart. As a result, it is safe and convenient for the Audi system to shut off the engine as soon as it hits 13 mph while slowing down. This also allows the engine to be shut off for longer than a normal start-stop system would. According to Audi, it's also fast and smooth enough that the engine can be shut off during coasting for up to 40 seconds between speeds of about 35 and 99 mph.
There's one other advantage to the way this Audi system is designed. Because the starter capability is included in the belt-driven alternator, it's hypothetically able to be used on any vehicle with any powertrain, provided there's a 48V electrical system. It can simply be bolted onto the front of an engine, instead of needing a whole motor sandwiched somewhere between the engine and transmission. So don't be surprised if you see it show up on other Audis in the near future.
This feature will be standard on all next-generation Audi A8s. The car will be fully revealed on July 11.