An explanation of the Koan answer
It's again about "oneness". Like in Case 12, somebody calls, somebody answers.
This time a servant answers his master's call. The servant was tradionally seen as being so close to the master, that he answerd instead of the master.
Call and answer are not different but "the same" or "one". In another case Joshu addressed two monks identically they answered identically. But Joshu seemed to distinguish these identical answers, just to confuse his listeners.
Stories playing with "oneness" are quite common in Zen.
The background story of this Koan
In medieval China the emperor sometimes honoured a master with the title "national teacher".
These masters were often seen as especially able teacher.
Kokushi was highly respected although the story is quite conventional.
Chu, called Kokushi, the teacher of the emperor, called to his attendant: "Oshin."
Oshin answered: "Yes."
Chu repeated, to test his pupil: "Oshin."
Oshin repeated: "Yes."
Chu called: "Oshin."
Oshin answered: "Yes."
Chu said: "I ought to apologize to you for all this calling, but really you ought to apologize to me."
The original Chinese Goang
The National Teacher called his attendant thrice.
The attendant responded thrice.
The National Teacher said,
"When you first came I was ready to say I was disappointing you,
instead it is you disappointing me."
Traditional Commentaries and .... Poems (Gata)
When old Chu called Oshin three times his tongue was rotting,
but when Oshin answered three times his words were brilliant.
Chu was getting decrepit and lonesome, and his method of teaching was like holding a cow's head to feed it clover.
Oshin did not trouble to show his Zen either. His satisfied stomach had no desire to feast. When the country is prosperous everyone is indolent; when the home is wealthy the children are spoiled.
Now I want to ask you: Which one should apologize?
The National Teacher called thrice, and his tongue fell to the ground.
The attendant responded thrice, and he spit out the light of harmony.
The National Teacher was old in years and his heart was lonely. He pushed the ox’s head to eat grass.
The attendant did not consent to shoulder the responsibility.
Delicacies do not hit the spot of the satiated person. Just say, within this case at what spot was he disappointed? Only in the nation is a gifted scholar honored. In rich families the small children are pampered.