In this 13-string zither, there are two middle bridges dividing each string into three playable segments. Two of the segments are equal in length; the third is half as long. This amounts to two segments at the same pitch, with the third segment an octave higher. Naturally there’s a lot of transmission between the segments across the bridges, and with the unison and octave relationships, there are plenty of shared resonances arising whenever any one of the segments is plucked. The fact that these tunings are rarely perfect means that there are chorusing effects at play in the interactions, and altogether this creates a distinctive tone.
In this audio clip, 2+2+1 is joined by the Sour Bells. In the video linked below, 2+2+1 is the third of several instruments described.