Hey skaidiver,
Facedown Detection is an inherent feature of iOS 9, and you don’t have to turn it on or off. You can find more information about it here:
Foundation
• Longer battery life
◦ Up to one hour of additional time before you need to charge your battery
◦ Facedown detection for turning display off when not in use
◦ Low power mode optimizes device performance to extend battery by up to an additional three hours
• Software updates require less space to download and offer an option to install later
• iOS apps and user interface now use Metal to deliver faster scrolling, smoother animation, and better overall performance
• Improved security with two-factor authentication support and a default 6 digit passcode support for Touch ID devices
iOS 9
https://support.apple.com/kb/DL1842
Thanks for coming to the Apple Support Communities!