Facedown detection on iPhone 6 with iOS 9 Apple highlights that an iPhone — and presumably other iOS devices — will use light and proximity sensors to determine whether it's laying face-down, and stop the screen from turning on even when notifications come in.

It does not seem to work on my iPhone 6. Do I have to enable low power mode for this to work?
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