“The axes are invisible; the chops are for real.” The art of air guitar centers on the elusive quality of “airness.” Covering the U.S. Air Guitar Championships in Los Angeles and the World Finals in Oulu, Finland, the film appreciates the enthusiasm of the fans and the weird sweetness of the performers. It’s also a friendly send-up of the genre of “count-down” competition documentaries. Proponents of air guitar knowingly acknowledge its metaphysical nothingness while a blistering soundtrack of guitar-heavy hits keeps the focus on rock and roll.  NK