Double-tapping the home button

[rhyme]

I notice that if I double-tap (just touch, not press) the home button on my iPhone 6S while an app is open, the app slides down part-way, revealing... nothing. An empty dark grey area. Any idea what this is supposed to be for?

 

[MrLoL]

It's reachability, it's meant to bring the upper side of the screen down in order to tap on something that you can't reach one-handed. I personally use it a lot

 

[powerocool]

It's called Reachability

http://www.imore.com/how-use-iphone-6-and-6-plus-one-handed-reachability