Nascar Truck Series Spec Question?

[CAPTAIN]

OK, I've noticed that the nascar trucks have the rear bars of the roll cage going into the bed. Now it looks like they have plexiglass or lexan mounted to the bars in that opening. My question is this. What is the point of that? I'm assuming it helps smooth out the airflow over the truck, correct? Kinda acts like a foil of sorts?
How are they mounted? Thickness?

Anyone got a clue?