The slotted board will obviously change flow direction - one only has to look at it to see it will send flow upwards. Remember the flow hitting that board will be moving from inside to outside as well as from car front to car back. So the flaps will produce a number of similar vortices. Similar vortices combine to produce a large effect which is what they'll be trying to achieve. This whole area is about setting up the flow to limit movement under the floor's edge. Roll up a strong vortex and run it along the top of the floor edge and you can reduce the movement of air from above the floor to under the floor - the so-called "sealing of the floor".
Turbo says "Dumpster sounds so much more classy. It's the diamond of the cesspools." oh, and "The Dutch fans are drunk. Maybe"