NACA ducts - how do they work?

Anyone know how a NACA duct works and what it is supposed to do? I've
seen them used to expel air from a bonnet and as a scoop to catch air
for induction. They've been used in both orientations, i.e. narrow end
backwards AND forwards. I've also seen the narrow end opened OR the wide
end open. I'm sure I read somewhere that they utilise a venturi type
effect.
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