All of these teenagers will at some point have to decide between remaining in the amateur system or making the jump to professional fighting.

"It's a way out, a chance to improve the lives of themselves and their families," Yi said. "So they are willing to 'eat bitterness' and train very hard. Only a handful can make it to the top of the pyramid. Years of hard training may end in vain. In a country where money has become a religion, these boxers will have to fight very hard against the financial pressure from their family and society."

Yi expects boxing to grow in popularity in China. State boxing regulators have begun working on ways to promote the professional version of the sport on a larger scale, she said, pointing out that weekly rebroadcasts of pro fights already enjoy an audience of 30 million.

The film has enjoyed a warm reception in the US. The New York Times called it a "remarkably tender portrait"; The New York Post deemed the film "Rocky, Asian style."

For Chang, boxing is a microcosm of life,

"So often for boxers, it's a story of impoverished people fighting their way out of poverty," he said. "It's a classic tale. I was interested in taking that Western idea and placing it in the realm of a Chinese story."

This rags-to-riches element of the documentary rang true with Tyson, who Chang met in China while working on the film. He recounted meeting the American ex-heavyweight in 2010 in Tianjin, where Tyson had been hired to serve as a temporary ambassador for the sport. For three days, Chang camped outside Tyson's luxury hotel, making inroads with members of the boxer's entourage. On his last day in China, Tyson requested three items: a toenail clipper, a pomegranate, and a Shaw Brothers kung fu movie. His entourage asked Chang to help; the director failed to find any of the items but he did present the boxer a copy of Up the Yangtze. He later showed Tyson footage from "China Heavyweight".

"He was moved by the footage and said that these were the same experiences he'd gone through as a poor kid fighting his way up and out," he said. Tyson, who has a tattoo of Mao Zedong on his arm, became interested in China while incarcerated in the mid-1990s on a rape conviction, Chang said.

The demo included a clip of Don King, the legendary boxing promoter and former manager whom Tyson sued for $100 million in 1998. When Tyson finished watching the footage , he told Chang: "Don King is Chiang Kai-shek and I am Mao Zedong, and I'm going to kick him out of China."

kdawson@chinadailyusa.com