Yeah, there was an episode that talked about this joint. If you look a
the end grain of the piece with the tail cut in it, it's a square cut
which creates the illusion that it's a square cut all the way back to
shoulders like a normal dovetail would be. That's impossible though.
What's really going on is that the sides of both dovetails are beveled
so that where the dovetail attached to the rest of the board, the sides
of the tail are at a 45 degree angle. The board simply pushes straight
in from the back. Between the two dovetails is sort of a V-shaped
valley with a matching one on the other piece. It's difficult to
describe, but easy to understand once you see it.