The cowl hood will pull cooler air into the engine bay at the base of the windshield.

 

It lets hot air escape through the back of the hood.

 

only at idle, not when you are moving.

Right! when you are driving it the flow of air will go over the hood and "sucked" into the engine at the end of the hood (near the windshield cowl)
It's all aerodynamics...

However we are talking about a truck here, so I havne't a clue how the air flows over it. (proabaly like a brick wall)

 

I'm curious as to how your scoop handles a heavy downpour of water, does it dump the water over the open air filter, or does it drain off before it reaches the filter?

I too was conserned with this one day when we got a lot of rain..I checked when I got home and the fiilter was fine and only a little tiny bit of water was coming out. Actually none even hit the filter or even droped down it just got pushed back to the rear. Remember that there is always air under the hood mooving aft so it's not like the water (very little) is just going to drop on the fillter like a water fall because there is air hitting it and mooving it to ther rear.