I’m a little hesitant to post this because it’s in Korean — most of you speak English — and I can’t track down the author’s name. But I think this is one of the most interesting explanations of the iOS 9 multitasking interface. Per Bing’s translator [sic]:
Why, then, do the same in the plane of the app is not listed in the GUI switch to overlapping method applied with perspective? This leads to can be found in the 3D touch itself. Light objects than when moving heavy objects might have more power when you move, Apple apps screenshots page in-app has given a “heavy” feeling than a page remains to be seen. This is the sense of the weight of the user for the hierarchical levels in the structure in-app screenshots of the app itself rather than the page, the page higher stage allows us to know intuitively that there is. In other words, Apple has introduced a touch gesture iOS a new level of intensity over from 7 to more users by easy to understand and to be able to experience the UI changes.
If I’m reading this right, what the author is saying is that the 3D Touch feature of the iPhone 6S makes it feel like you’re pushing older apps back and away from the display, granting them “weight”.
This is worth taking a look at if only for the excellent diagrams.