The bokuto slices through the air, as tough and slender as the 25-year-old woman holding it.

“I’m the only white girl in the world who does this,” says actress Samaire Armstrong, who uses the wooden sword to exercise on set. “It centers me. When you get your left hand onto the butt of the sword, all of a sudden there’s this whole other power source.”

One wouldn’t suspect that the petite star of television’s “The OC” and the upcoming feature “It’s a Boy Girl Thing” was a seasoned martial artist, but since the age of 4, Armstrong’s trained in judo, karate and her specialty, Yagyu Shinkage-ryu — Japanese sword training.

One of the most renowned forms of swordsmanship, Yagyu Shinkage-ryu has been passed from generation to generation since the 16th century. There are no schools. No tournaments. It can be learned only from the Yagyu family itself, still living in Japan. Armstrong picked up the techniques while training in Japan with her father, who was studying hoplology, the study of combative behavior.

“(Yagyu Shinkage-ryu) is basically the techniques that samurai used to survive on the battlefield,” Armstrong explains, “but it can be applied to everyday life. It’s about being aware of your environment, being aware of the space. They’re not techniques to kill, although you know how if you have to.”

Armstrong, a steadily working actress, no longer trains as intensively as she once did, but she frequently works out at home with a batto, or “live blade,” and a 12-foot yari spear.

“Sometimes when I’m studying my lines,” says Armstrong, “I’ll go through the lines in my head with the kata, the series of movements you do when you train. Each beat will end on a movement, so it ties in with an intent. I’ll block my whole scene to the kata … and you start to remember it.”

This, of course, is the true purpose of Shinkage-ryu — not to give people the ability to kill but to arm them with the skills necessary to navigate the battlefield of everyday life.

“It has given me great confidence,” Armstrong says. “Wherever I am, I know I can survive, mentally and physically. That’s something I don’t think you’re born with, but you can teach yourself. You just have to believe.”