Seymour Duncan Double Back Compressor SFX-09 Simply put, a compressor makes quiet passages louder and loud passages quieter. They're used to make you sound more "professional" by providing smooth dynamics and enhanced sustain. But most other compressors can get in the way of your guitar's natural transients and feel. Not the Double Back™. This feature, which gives this pedal its name, allows some of the uncompressed sound - specifically the initial "snap" of the string - to "double back" into the compressor output, thus preserving the essential essence and sensation of a string being plucked. And it's extremely quiet.