It hints that at the app list, which is a nice touch given that they killed the gutter and the arrow teasing you to swipe over to access the app list. It also ties in well with the full bleed concept Metro does so well and makes it feel like the phone is a window to so much more than what's displayed at a glance. I like it.
Nice way to hint at the app list without having the gutter sitting there all the time. Also looks cool. Hope the animation is very short when not slowed down for the GIF, or else it will get tiresome with time.