In/out design

In order to achieve the correct pressure difference to move the right amount of air across the head, each vent must have an intake and an outlet. Any disruption of this symmetry compromises the ability of the helmet to properly ventilate. So the key to ventilation is not the number of vents you have (in fact, too many vents can have negative consequences) but the arrangement and balance of these vents on the helmet. 

That is what In/out technology does, it links each intake vent to an outlet vent in order to create and maintain a good airflow to cool you off, but not also keep you warm.