One of the many new features introduced with iOS 9 is facedown detection. Gautam Prabhu, writing for iPhoneHacks:

In case you went through iOS 9’s release notes, you may have noticed it includes a new feature called Facedown detection. The feature along with other refinements helps in improving battery life by an additional hour as it turns off the display when not in use.

So one of the top iOS 9 tips and tricks is to put your iPhone facedown to ensure that the display doesn’t light up when you receive a Notification, which in turn helps in improving battery life.

Great idea. As it turns out, facedown detection does not work on all iOS devices. If you are interested in the details, check out Gautam’s follow-up post. It’s all about the motion co-processor.