It's peculiar, and none of the answers thus far seem particularly satisfying, given the hype that has surrounded this thing. But, I don't have anything better, so.... :-"
Whatever the case may be, here's some mental fodder. (Note: only the top panel in the first image depicts W07. The rest are W06 from last year's post-race test in Brazil.)
I wonder if it's just skimming off air flow that otherwise loses enough dynamic pressure to become relatively stagnant when allowed to continue uninterrupted along the original streamline.
Note: offset pitot tube