The Palio di Siena is a race like no other. Held biannually at the Piazza del Campo in Siena, Italy, it is a race not only of skill but of bribery, manipulation and corruption.

Seventeen jockeys, each representing a medieval district, ride bareback, equipped with a stretched and dried ox penis, which is used to whip each other and their horses. The winner receives a monetary award from the district he represents, but oftentimes other riders are paid off to lose—and to take their opponents down with them, by any means necessary.

But half-German, half-Italian jockey Giovanni Atzeni is a rare player in the corrupt game. Although he remained mostly invisible during his first few summers competing in the Palio, other jockeys and trainers alike began to take notice of the 29-year-old’s undeniable talent and ambition.

So did filmmaker Cosima Spender. Her documentary “Palio”—which will screen this weekend at the Hamptons International Film Festival—was never meant to focus on Mr. Atzeni. But the David-and-Goliath battle between him and his former mentor, Luigi “Gigi” Bruschelli, cemented the young, humble jockey as the heart of the film.

“For Giovanni, it’s more important for him to assert himself as a player, as one of the top jockeys,” Ms. Spender said last week during a telephone interview, taking a brief break from the action at the Vancouver International Film Festival. “He is choosing to follow the best horses and not necessarily go for the richest district.”

Initially, the documentary focused more on the hierarchy between jockeys, but the natural epic between Mr. Atzeni and Mr. Bruschelli steered the film in a different direction, Ms. Spender said, instead focusing on the jockey’s transformation into a bona fide threat.

“To me, [Mr. Atzeni] represented all the aspirations of the Palio jockey. We saw him change during the process. When I first approached him, even in my research, he was quite insecure,” the director said. “He was touched that we decided to follow him, but he didn’t quite believe it. Slowly, we started to see him gaining confidence. We saw the innocent become more experienced.”

Mr. Bruschelli presents a Hollywood-worthy adversary to Mr. Atzeni. He won 13 Palios over a 16-year period—a record second only to famed jockey Andrea Degortes, who is known as “Aceto.” While he may be approaching the end of his career at age 46, Mr. Bruschelli’s power fails to diminish, Ms. Spender said. With such a long streak of victories, he’s in a prime position to buy favors and control the game—and he knows it.

“He’s incredibly respected for being the best strategist and he knows how to play the Palio game like no other jockey,” she said.

But unlike Western sports, the jockeys are not revered as heroes. They are considered selfish and money-hungry, willing to sell out their district for personal gain. Although some remain iconic, much like Mr. Bruschelli, a loss can mean backlash from thousands within the losing districts.

“The jockeys are both loved and despised,” Ms. Spender explained. “They’re in the eye of the storm and yet they’re mistrusted. It’s an interesting lesson in human behavior.”

Many Palios even have a violent end, she reported, citing that it is common to see the crowds turn on the jockeys.

“There is a lot of violence, partly because the city is, economically, in a fragile place and there’s a lot of unemployment,” Ms. Spender said. “For the young, there aren’t many jobs around, so they are frustrated. And at the Palio, emotions are running very high, everyone’s drunk too much, they’re exhausted, they haven’t slept for four days. And so, inevitably, fights break out. They are part of it.”

One former jockey in the film describes how he was forced to fight for his life after one particularly heated Palio. However, any other time of the year, the city is surprisingly calm, according to Ms. Spender.

“I mean, nothing much is going on and it’s as if everyone is saving their pent-up emotions for their days at the Palio, and it’s very cathartic,” she said. “You let things out. It’s almost accepting this duality in nature—the good and the bad—and on those days, everything comes out.”

Over the course of the film, Mr. Atzeni and Mr. Bruschelli become their own opposing forces of nature, epitomizing the competitiveness of the sport, where a student can rise and a master can fall in the blink of an eye.

“For [Siena], the Palio is a metaphor for life and it’s about trying to control your life, humans trying to control everything in their life,” Ms. Spender said. “But there’s an element of luck that goes into it.”

“Palio” will screen on Saturday, October 10, at 4:45 p.m. at Regal East Hampton Cinema and Sunday, October 11, at 11:30 a.m. at the Sag Harbor Cinema, as part of the World Cinema program during the 23rd annual Hamptons International Film Festival. Tickets are $15. The film will be released in theaters on Friday, November 6. For more information, visit hamptonsfilmfest.org.