The BMW air curtain is a very subtle corner canard as compared to very extravagant front corner versions from the Japanese racing aftermarket. These external examples are corner box canard/splitters. Note the horizontal winglets are angled flat, collecting front air pressure flow and narrowing it down just before the well opening, probably with similar hopeful results.
http://www.gtrblog.com/wp-content/up...02-540x405.jpg
Other concept cars with these devices.
http://theblogaboutcars.com/wp-conte...citroen-GT.jpg