Sgt. Casey Day was skeptical. The chief of his rural Northern California police department wanted him to find out if an ancient martial arts weapon made famous in 1970s Bruce Lee movies could be used to subdue unruly suspects. But it only took a few days of training with nunchucks to win over Day. The weapon—two solid sticks of plastic attached by a foot-long nylon cord—was recently added to the Anderson Police Department’s arsenal. The department of 20 sworn officers about 200 miles north of San Francisco joined several other U.S. law enforcement agencies that use nunchucks as “less than...