In its purest form, skateboarding is a simple act of gravity. Point your board downhill, step on, and let Newtonian physics dictate what happens next. Pretty basic stuff, really. Unless, of course, you were standing on your skateboard during the mid-1970s at the top of Southern California's infamous Signal Hill. From there, a downhill speed run wasn't so much an easy surrender to a grand force of nature as it was a potentially deadly leap of faith.

That dangerously steep drop -- with all its broken bones, bloody bruises and high-performance breakthroughs -- are the subject of "The Signal Hill Speed Run," a feature-length documentary now making the film festival rounds.

The film gets going in 1975, when Jim O'Mahoney -- described as the P.T. Barnum of skate contest promoters -- organized the first Signal Hill event after getting a call from producers at "David Frost Presents the Guinness Book of World Records," an ABC "Wide World" special.

Having grown up surfing, skating, and hang gliding in and around his native Long Beach, Calif., O'Mahoney -- who bombed Signal Hill on his bike as a boy -- was hardwired for adrenalin and he had no intention of organizing something cute and surfy.

Let's take Signal Hill from the top, he told ABC. Winner takes all.

O'Mahoney, who now curates the Santa Barbara Surfing Museum, spread the word throughout the surf and skate shops, and on the day of reckoning, all but two of the invited skaters -- a handful of the best downhillers in the small world of competitive skateboarding -- refused to take the drop. It was an experience of full commitment, "like Acapulco cliff diving," Santa Cruz Skateboards' Tim Piumarta says in the movie.

Topping out at 50.2 mph down the almost 30-degree incline, local surfer-skater Guy Grundy won that inaugural contest and instantly became the world's fastest skateboarder.

Then the gloves came off.

For the next three annual installments, O'Mahoney's contest -- which often resembled a midday keg party flanked by idling ambulances -- grew exponentially in terms of the number of competitors, advancements in equipment and technique, and trips to the closest emergency room.

Aside from the carnage, the film captures skateboarding's early contributions to today's so-called extreme sports, including the heart-wrenchingly heroic runs and crash by 21-year-old world champion hang glider Tina Trefethen, who was seriously injured and paid tragically to become the world's fastest female.

"The movie does a great job of showing what came out of Signal Hill," says Jack Smith, curator of the Morro Bay Skatelab Skateboard Museum and publisher of "The Skateboarder's Journal," who started racing in 1975. "For downhillers, this movie is our 'Dogtown and Z-boys.'"

The next scheduled public screening of "The Signal Hill Speed Run" will be in March at the San Luis Obispo International Film Festival.