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Sorry, what kach22i said. It's an external aero duct.
Occasionally, someone will concoct a plan to run a duct from the front stagnation point to the back to raise the base pressure, but of course skin friction would kill it so there's nothing to be gained. But, IMHO, a short, large duct can be useful externally. I think the Ferrari is a good example, diverting air sideways before it piles up under the windshield. I don't know if it would do much downstream.
The GTI W12-650 is essentially a bubble-top coupe body inside of a hatch-back. That gains plan taper and an integrated wing. It diverts 5-10x more air, and there is no downstream to make claims about. So I think it is a better example.
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