On iOS, you switch apps by double-pressing the home button, then swiping. Previously on iOS 9, when you swiped, the windows would slide across the screen, and it was easy to swipe too far, and have to swipe back to get to the app you want.
In iOS 9.2, Apple made a subtle change. When you swipe now, one window slides out of the way, but the stream of windows stops on the next one, instead of continuing, potentially past the window you want to tap. It’s as if the windows snap into position as you swipe.
This is easier to understand when you try it out. So go ahead, double-tap that home button, and swipe; you’ll see what I mean.
Update: it turns out that not everyone sees this. This new app switcher behavior only occurs if you have the Reduce Motion setting turned on in the Accessibility settings. I have Reduce Motion turned on to eliminate many of the swimmy animations in iOS which serve no purpose and are distracting.