One of the features of iOS 9 I was most excited about was the ability to use two apps at once. When you are in an app, swipe left from the right side of the screen and a drawer opens. These are apps you can open in addition to the one you started in. It turns out that there are some limitations. You can't open just any app, just certain ones. It looks to me like the ones you can open in this reel are just the ones that are native to the iPad. That's a good start, but I hope more will be added.
Now to the Keynote treasure part: When you are presenting in Keynote on your iPad and you slide open this drawer, it doesn't get projected! So I can have a slide up - maybe my students are copying some notes before we talk about them or they are solving a problem - and open these other apps so that I can use Safari to take attendance or use the timer to keep us all on schedule. The students see the slide but I can open the second app and keep it hidden.