The NACA duct is a very special air inlet device that allows cooling air to be pulled into ducted chamber with a minimal flow disturbance over the larger mechanical body. Unlike, say, a hood scoop or a snorkel which stand up in clean air for a ram effect but leave an effluent wake of dirty air in their path - the NACA duct is a submerged inlet that uses a shallow ramp and curved walls as a depression within the larger mechanical body. The ramp allows air passing along the body to slip down into the duct and the curved walls help direct the air flow AND increase the air speed. Air's velocity must increase through the constriction... cause you can't leave out the equation of continuity & pressure must decrease because of our friend Energy and the conservation there of.
More simply, the design works because, perhaps more importantly, the curved walls also develop counter rotating vortices which in turn push away the slow moving boundary air adhered to the mechanical body and suck in the faster moving air.
Fortunately for race car design these aerodynamically efficient ducts are excellent for charge cooling and venting air intakes for mechanical and human components. NACA duct are however minimally effective for significant air volumes and are therefore rarely used for either jet or large bore engine systems. That work is still left to the snorkels and scoops and their superior ram air effect.
Oh, and NACA stands for National Advisory Committee for Aeronautics, aka NASA's grandfather.
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