This little string contraption is played using a short length of threaded rod as if it were a bow. But being heavier, harder and more rigid than a violin bow, the rod imposes itself on the string like the slide of a slide guitar, with the point of bowing determining the string’s vibrating length. Thus you can vary the pitch by varying the bowing location. Typically the string segments on both sides of the bow will sound, creating dual pitches, but you may choose to damp out one side by weaving a cloth over and under the strings adjacent to the bridge. The instrument is quiet when played acoustically, but it includes a pickup.
In the two photos on this page, two versions of 4-40 appear. One is acoustic; the other uses a pickup.