Japanese Board Stretching

I wonder what the engineering specs on a joint like that is. Like what percent of the strength of the unjoined beam?
 
Back whe I was a painter I was working in Georgetown staining some work at a home that once belonged to Lisbeth Taylor and the new owner was having a in-law sweet built like this. I did not get much work done watching the guy work , no nails or screws. He was American and studied many years in Japan.
 
What I want to see is how they laid out and cut the joint. Way cool! And the little gap on the one end was for the disjointing wedge. Clever!
 
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