Microsoft engineer Goran P. wrote on Feedback Hub today that the Double-tap-to-wake feature is coming to Windows 10 Mobile handsets that support it. Microsoft did not provide a list of specific handsets that will be receiving this capability. At least one person with the Microsoft Lumia 950 XL reported that he was able to use the feature.
If you've never seen or used "Double-tap-to-wake," it allows you to awaken a sleeping handset by tapping twice on the screen. This way, you don't have to stretch to press on the phone's power button to make the phone operational. For most models, it is much easier to double-tap the screen than it is to gain access to the power button.