Ok, you guys can blame @HandyDan for this one, he sent it to me and its been driving me nuts.

 

 It appears that the joint on the ends and the sides of the logs are all the same. The 4 points curve up & down when looking at the ends and they fit in the mating grooves on the sides of the log.

I've circled the part that is confusing me. How is this cut? Is the curved cut inside the log, inside the edges, going in two directions to make the fit. 

In my mind I envision a compound cut. If you were to cut a diamond shape from a rubber ball, leave one end of the diamond shape curved like the ball and at the center invert the other end in the opposite direction, if that even makes sense. Maybe I'm just looking too hard. Thanks Dan

 

It is a compound cut but keep in mind they are stacked on top of each other not driven together.  They did a great job of hiding the joints.

Hope that answers your question.

 

Ok, this is basically what my brain couldn't see. 

 

           

Does Festool make a machine for this?

I would love to see the machine that did that. That is an impressive mortise and one very rigid construction technique. 

Paul

I have an extra one not in use right now if you want to borrow it....