If you set up narrow steel rods to be played like marimba bars, they work poorly because the rods lack sufficient surface area to project their sound into the air, especially for the low frequencies. But if you put a magnetic pickup under the bars, the fullness of their sound is revealed. The 30 bars here have been reshaped to bring the overtones in line harmonically with the fundamental. They are mounted both above and below the pickup in a way that deliberately gives them a less ringing, more percussive tone. They can be played with mallets or by finger-plucking the ends of the bars.
In the audio clip, Quarter-Inch is accompanied by the cubist bass, a classical guitar, and percussion.