In iOS 9.x, swiping through your running apps is a fairly swift operation, and it's easy to overshoot the app you want. In iOS 9.2, with Settings -> General -> Accessibility -> Reduce Motion turned on, swipes in the App Switcher advance through running apps one at a time.
While you need to experience it live to truly see the difference, I captured two screenshots that show some of what has changed. The first shows the App Switcher in iOS 9.2 without Reduce Motion activated:
App Switcher in iOS 9.2
Because the swipe results in a very fluid motion, there's a somewhat even overlap between apps. The second screenshot shows the App Switcher with Reduce Motion turned on.
App Switcher in iOS 9.2 with Reduce Motion Activated
In this mode, each swipe snaps to this view, with one app clearly front and center. It is precise and unambiguous compared to the App Switcher's normal operation.
Reduce Motion also reduces the parallax feature on the iOS home screen, and many believe it improves overall performance on your device. I don't normally use it on my iPhone 6s because I enjoy the slight parallax effect in iOS.
In addition, whether or not you like the App Switcher advancing one app at a time or lickety-split through multiple apps is entirely subjective. To that end, I'd personally rather have the option of swiping through multiple apps and sometimes overshooting it.
But, if you've been annoyed with the App Switcher in iOS 9, check out how it works with Reduce Motion turned on.
Kirk McElhearn first spotted the new feature and identified it as being a product of Reduce Motion.