iOS 9.2 double touch fingerprint sensor

I was messing around this morning and found that if I quickly double touch (NOT double-click) the fingerprint sensor on my iPhone 6S+ (iOS 9.2), the current app (or the home screen) slides down to the bottom half-screen, showing a blurred home screen background on the top half. It doesn't matter if I use a finger that the sensor recognizes. It goes back to normal after about 8 seconds or if you double-touch again, but I can't figure out what this mode is for?? Is this a bug or some hidden feature I haven't heard about yet?

It's a feature! It's so you can reach the upper part of the screen with you thumb. It's called reachability, for more info here.