The background story of this Koan
Bodhidharma sits facing the wall.
His future successor stands in the snow and presents his severed arm to Bodhidharma.
He cries: "My mind is not pacified. Master, pacify my mind."
Bodhidharma says: "If you bring me that mind, I will pacify it for you."
The successor says: "When I search my mind I cannot hold it."
Bodhidharma says: "Then your mind is pacified already."
This famous but cruel transmission story had unexpected and unintended consequences. Later monks tried to imitate Eka, not by cutting the whole arm but parts of their little finger. This was meant to show the monks unshakeable will to attain enlightenment.
The story has another reference, from 6th century, at the time of Bodhidharma came to China. It's Tan-lin's ("armless Lin") text "Treatise on the Two Entrances and four Practises", a rather short text about meditation concepts. He most probably lost his arm when bandits caught him with no possessions.
In the context of the Koan it means Eka like Tan-Lin even risked to loose an arm just to meet Bodhidharma.
What next?
The original Chinese Goang
Dharma faced toward the wall.
The Second Ancestor stood in the snow, cut off his arm, and said, “This disciple’s heart-mind has not yet been pacified. I beg teacher to pacify my heart-mind.”
Ma said, “Come here with your heart-mind, and I will pacify it for you.”
Ancestor said, “My searching for heart-mind is completed, and I’m not able to obtain it!”
Ma said, "I have finished pacifying your heart-mind for you.”
What next?
Traditional Commentaries and .... Poems (Gata)
Mumon's comment: That broken-toothed old Hindu, Bodhidharma, came thousands of miles over the sea from India to China as if he had something wonderful.
He is like raising waves without wind.
After he remained years in China he had only one disciple and that one lost his arm and was deformed.
Alas, ever since he has had brainless disciples.
Wumen says:The gap-toothed old Barbarian sailed on the ocean a hundred thousand li especially according to come here.
One can rightly say this is raising waves without wind.
After it was ended, he accepted and gained one particular man of the gate, and yet he was not equipped with the six roots.
Alas, Xiesanlang did not know four words.
What next?