Let's go on with the sexy striptease of our cars.
This time I am trying to explain the reason for the hole that connects the upper part of the bottom with the diffuser. The mechanism is the reverse of the "double diffuser" because air is added (and not removed) from inside the diffuser. The intent is to clean the turbulent flow (due to a too abrupt transition imposed by the rules). The mechanism is similar to that of the tangential flow that passes from the high pressure zone to the low pressure zone of a wing profile composed of several airfoils (see Koldskaal and Variante posts above).
The gain is small, but it can help: Cl*A + 3% Cd*A - 1%.
With the layout of my cooling system there is also a very small but welcome increase in flow, because the diffuser sucks air in the air outlet area.