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Very close Eric.

As the air passes over the bulge before the vent opening it has to speed up which creates a low pressure system (like how a wing works per Bernouli's Law), just in the cavity above the vent. The pressure here is lower than the air pressure in the engine bay and thus the air in the engine bay exits the engine bay through the vent in the hood as "high pressure fluids always move to a lower pressure area". So it's almost like the air passing over the hood, as you are driving, is sucking the air out of the engine bay, which helps drop the engine bay temp as the hot air is replaced by cooler fresh air from under the car and from through the radiator. So it makes the car run cooler, which is always good.