Many insects vary their song patterns to communicate different messages, but the underlying biomechanisms are often poorly understood. Here we report on the mechanics of sound production and variation in an elytro-tergal stridulator, male Dendroctonus valens bark beetles. Using ablation experiments coupled with high-speed video and audio recordings, we show that (1) chirps are produced using a stridulatory file on the left elytron (forewing) and a protrusion (plectrum) on the seventh abdominal segment, (2) chirps are produced by spring stridulation, a catch-and-release mechanism whereby the plectrum catches on a file tooth, and upon release, springs forward along the file, and (3) variability in chirp types is caused by introducing multiple catch and release events along the file to create regular interruptions. These results provide experimental evidence for the mechanics of elytrotergal stridulation, and provide insight into how an insect can incorporate variability into its acoustic repertoire using a spring-loaded mechanism.