The fictional generations of Chinese Zen-masters the lineage, was always undisputed until the fifth generation patriarch Hongren, who was supposed to lived in Huangmai, a legendary 7th century monastery.
His successor as sixth patriarch was Shen Xiu the head monk of Huangmai.
Two generations later, an obscure figure in Zen history, Shenghui, made up the story of this Koan to introduce Huineng as sixth patriarch instead of Shenxiu.
A clever move, for Shen Xiu was already dead and as disciple of the now sixth patriarch Huineng Shenhui became the seventh patriarch.
The story begins in Huangmai, with a poem writing contest. The winner would become the successor of Hongren, as sixth patriarch.
The headmonk Shen Xiu (Myo) loses the contest against Huineng (Eno), the story runs, but Myo wouldn't give in. Myo ran after Eno, who had aready left the monastery.
The Koan story describes Huineng (Eno) as being superior to Shen Xiu (Myo) who's unable to lift the robe of succession. Shen Xiu finally surrendered and asked to be taught by the much younger Huineng.
To make his story more authentic it was added to a manuscript (most likely) from a monastery in Southern China, called the Ox-Head-School, which became the a basic teaching of Huineng.
Even more importantly this manuscript was called a Sutra, the Platform Sutra of Huineng, the Sixth Patriarch.
For centuries this text was highly regarded in Chinese Zen as the only Sutra which didn't derive from Buddha's teachings.
Modern sinologists discovered Shenhui's "forgery". Even so, until today Huineng is regarded in Zen as sixth patriarch and Shen Xiu is more or less forgotten.
The Koan-Story according to the Platform Sutra.
When he became emancipated (enlightened) the sixth patriarch received from the fifth patriarch the bowl and robe given from the Buddha to his successors, generation after generation.
A monk named E-myo out of envy pursued the sixth patriarch to take this great treasure away from him. The patriarch placed the bowl and robe on a stone in the road and told E-myo:
"These objects just symbolize the faith. There is no use fighting over them. If you desire to take them, take them now."
When E-myo went to move the bowl and robe they were as heavy as mountains. He could not budge them.
Trembling for shame he said: "I came wanting the teaching, not the material treasures. Please teach me."
The sixth patriarch said: "When you do not think good and when you do not think not-good, what is your true self?"
At these words E-myo was illumined. Perspiration broke out all over his body. He cried and bowed, saying: "You have given me the secret words and meanings. Is there yet a deeper part of the teaching?"
The sixth patriarch replied: "What I have told you is no secret at all. When you realize your own true self the secret belongs to you."
E-myo said: "I was under the fifth patriarch many years but could not realize my true self until now. Through your teaching I find the source. A person drinks water and knows himself whether it is cold or warm. May I call you my teacher?"
The sixth patriarch replied: "We studied together under the fifth patriarch. Call him your teacher, but just treasure what you have attained."