Double tap to wake was a feature built-in to Windows Phone 8.1 that allowed you to turn on your phone screen simply by double-tapping it. In Windows 10 Mobile, this feature didn't get ported over, which meant you needed to press the power button to turn your screen on. If you upgraded from a Windows Phone 8.1 device, your device should still work with double tap to wake on Windows 10 Mobile.
At the end of April, Gabe Aul tweeted that if users want double tap to come to Windows 10 Mobile, they'll need to upvote it.
Since that tweet, we didn't hear anything else mentioned about bringing the feature over to Microsoft's new mobile platform, until now. A Microsoft engineer, Goran P, replied to a couple feedback requests for the feature, stating that:
We are working on adding the support for double tap to wake for devices that support this feature.
Whilst Goran did not explicitly mention the devices that could support double tap to wake, it appears that both the 950 and 950XL will support it. Gabe mentioned the 950 specifically in his tweet, and Goran replied to a specific request for the feature to make its way to the 950XL:
Providing Microsoft manage to get the feature in a working state, we should begin to see it rolling into the Insider builds shortly, as part of the upcoming Anniversary update for Windows 10.
Source: Feedback Hub via MSPowerUser