Facedown detection on iPhone 6 with iOS 9
The motion coprocessor setting are what govern this battery saving option.

Settings > Privacy > Motion &amp; Fitness > Fitness Tracking 

On setting allow the motion coprocessor and proximity sensors to shut off the screen if they detect a face down orientation.

You can test the sensors by checking motion in the health app for step count (for instance) and by making a call and testing the proximity sensor turning off the screen when it is covered.

See this nice article for more information: http://www.iphonehacks.com/2015/09/why-facedown-detection-feature-in-ios-9-doesnt-support-iphone-5-iphone-4s-ipads-and-ipod-touches.html