Why iOS 9’s Facedown detection doesn’t work on iPhone 4s, iPhone 5, iPad and iPod touch
Facedown detection stands as one of the underrated features in iOS 9’s rich arsenal of power-saving tactics designed to help extend your run time by up to an additional hour.
Facedown detection avoids turning the display on when a notification comes in and your phone is put facedown. But as it turns out, Facedown detection doesn’t work on all devices compatible with iOS 9 and there is a good explanation for that why.