Wickerbill, or Gurney Flap. It's a vortex generator, a small-scale localized feature that 'pulls the air around a corner' to reduce the size of the wake.
Vekke's Solidworks CFD thread shows that a reflex curve can have similar effect.
I'm favoring the splitter for my XFi. I have a blank cut out of 1/8" ABS for when the motivation strikes.