[puddleglum]

I'm hoping to do a belly pan this summer and I'm wondering how to deal with the rear bumper. Here is a pic. of the stock bumper cover.

Iv'e noticed a similar grooved design on several other hatchback and SUV type cars also. My question is, is this just a design element or is there some aero. advantage to it? Is this supposed to help reduce the wake behind the car by pulling some of the underbody air up to fill the wake? It looks too rounded to me to do any good. I'm wondering if it would be better to run the pan up to it and leave the rear as is, or try to extend the pan past this to create a lower rear spoiler? Any advise is appreciated.