Web description NACA duct
"How do NACA ducts work?
NACA Ducts use the air attached to the plane or car and suck it inwards. The air hits the gentle ramp angle and the curvature profile of the walls and creates counter-rotating vortices. The NACA Duct is the smart designed shape that tricks airflow with the least amount of drag, that would eat fuel and speed"
And one of the most effective/efficient way to split off surface air with absolute minimal drag penalty.