UISimulatedApplicationResizeGestureEnabled (iOS 8 iPad split-screen multitasking debug) (video)
The code is demonstrated in iOS Simulator using a mouse, thus the awkward two-finger swipe gesture.
SpringBoard can run apps 1/4 size, 1/2 size, or 3/4 size. It's said that the feature isn't quite ready yet and that's why Apple didn't showcase it at WWDC.
Take a look at the video below and please follow iClarified on Twitter, Facebook, Google+, or RSS for updates.