As I understand it, with DFD, the camera knows which Panasonic system lens is mounted, and compares a database of that lens' "blur" (or defocus) characteristics to the scene that it "sees" through the glass. It uses that comparison to calculate the distance to the subject and set the focus of the lens. With the GH4 's quad core processor, this process is almost instantaneous - advertised at 0.07 sec.
As the video demonstrates, this is noticeably faster than the GH3 - and may rival the awesome autfocus performance of the Canon 70D.
It will be interesting to see which lenses have their defocus characteristics programmed into the camera at rollout. It's almost certain that Panasonic micro 4/3 system lenses will be programmed in. How about Oly m.Zuiko lenses? Sigma micro 4/3 lenses? Classic 4/3 lenses with the MA1 adapter? If non-Panasonic lenses aren't in the database at the start, Panasonic should get them in as soon as possible. It would be a shame to limit this capability to Panasonic OEM lenses only.