When I installed iOS 9 a couple of weeks ago on my iPad Air, I discovered the new multi-tasking feature. Interesting.
If you're in one app, and you swipe from right to left from the edge of the screen, depending on the app, a small tab will appear on the right edge.
If then you touch that tab and swipe it to the left, another app will be running in an overlaid section of the screen. With some apps however, the first swipe jumps straight to the overlay.
If you want to run a different app in that overlaid section, swipe down from the top, and an icon list of already open apps will appear and you can scroll up and select the one you want.
There's lots of overlaying and transparency there. I don't think every app will run in the top section of the screen, they need code changes to support this feature. Most of my apps have been updated to support this overlay feature, so that shouldn't be a problem.
I do a lot of reading with the app Marvin, and the motion I make to turn the page is exactly the same motion used to start this "multi-tasking" thing. After ten minutes of annoyance, I found the setting that turns off multi-tasking. Do not want.
Tonight, I had another look at it. Maybe it could be useful? I don't know, but I'll play with it a bit. If it gets irritating again, I'll just turn it off again.