The tab pops in the small hole and holds the the rubber against the open bigger square hole. When water comes down the rocker it pushes the flap open and drains the water. There are other places where the water can leak out from the cowl. Did you download the pics of that Camaro project I have under my goggle account?
Also, you will hear from a lot of guys to sand/media blast your car before doing any major welding. If there is any weak steel on your car body the blaster man will find it. 5-10 times more bad metal exposed after blasting.