Classic lawn edging air dam.
I will freely admit I have not tested ANY of these mods so far, and don't intend to.
In my limited experience, the air dam has a chance of making things slightly worse rather than better. I saw that on my 2nd gen Prius (tested). But aside from that car, all other similar air dams that I've tested on 2-box cars like the Fit have shown an improvement in either coastdown times or A-B-A MPG runs.
So I'm rollin' the dice and going in blind! And mixing metaphors!