Developer Adam Bell has hacked together a demo of the popular first-person shooter game Doom running on an iPad in iOS 9's Picture-in-Picture mode. A video shows him opening apps and performing other functions on his iPad, while a demo of Doom runs in a smaller window.
Bell says that his iPad isn't jailbroken, and he's not using any kind of process injection. His method involves the use of public developer APIs, which means there's theoretically nothing stopping devs from implementing this in their apps, although it's unlikely Apple would approve it.