Who is Tang Shunsan from The Rape of Nanking and what is their importance?

Tang Shunsan was a survivor who tells his story of a killing contest. Tang was taken with a group of hundreds of Chinese to the outskirts of town, bound, and forced to stand over a mass grave. He witnessed Japanese dogs feeding on the corpses nearby. The soldiers then began beheading the prisoners one by one in teams, competing to see who could kill the fastest. One woman, clearly pregnant, began to fight, only to have her intestines and fetus ripped from her body. Tang survived only because the team appointed to his section began slitting throats instead, and did not notice when the victim ahead of him fell backwards, pushing Tang into the grave unharmed. Soldiers later came to stab randomly into the pit to ensure all were dead, and Tang sustained five bayonet injuries. He survived only because his friends sneaked to the pit later that night to retrieve his body, only to find him alive. Tang was the only survivor.