Hey, look! Sean doesn't know what to call moves again. While we wait for ReverendKain and others to school me in the comments, I'll tell you that WWE calls it a between-the-turnbuckles dive DDT.
And it doesn't matter what you call it, it looks pretty cool in slow-mo, as you can see in this Vine of one he gave Samoa Joe, taken by Tool guitarist Adam Jones from the Jan. 27 taping at Full Sail Live and shared by Triple H:
A ringside view from @adamjones_tv of #SamiZayn vs. #SamoaJoe. @wwenxt #NXTTakeOver
It's also cool - if kayfabe busting - because you can see how Joe preps for and helps with the execution of the maneuver. It takes two to tango, after all.
Watch it in preparation for seeing it on the main roster, and possibly WrestleMania, soon.