From the MSPU story:
Microsoft is finally adding Double Tap To Wake to the Lumia 950 XL. The company quietly rolled out a new firmware update for the Lumia 950 XL via the Windows Device Recovery Tool, which brings the new feature for the device. This is a feature a lot of users have been asking for, and it’s very nice to see Microsoft finally adding this feature. At this moment, the update doesn’t seem to be available for all Lumia 950 XL just yet, and it isn’t available for the Lumia 950 or Lumia 650 just yet either. It seems like Microsoft has added the new feature with the firmware version 01078.00053.16236.350xx (varies depending on your region). Keep in mind that the feature may not be available on your Lumia 950 XL even if you have the latest firmware, which is a bit weird – but hopefully Microsoft will roll it out to more devices soon.
It's not clear which models will receive this update - the 650 might not make the cut. I'd expect the likes of HP and Acer to have something similar up their sleeve for their flagships now that the code is available to enable it under Windows 10.
Note that if you go via the Device Recovery Tool then you'll lose whatever's currently on your phone - including any Insider builds, meaning something of a major headache however you decide to rebuild. My advice would be to be patient and wait for this firmware to be available as a simple Over The Air update with no data loss and no hassle. After all, you've waited this long for DDTW.....(!)