The FX80B Compressor/Sustainer reduces the dynamic range of the input signal from an instrument. Dynamic range is the difference between the loudest and quietest portions of an instrument's sound. By "compressing" or squeezing the loudest and quietest portions of the instrument's sound into a more compact range, you can control the signal better. Since the loudest and quietest portions of your notes are compressed, your notes sustain longer, the sound seems tighter, and you can help avoid clipping going into other effects or your amplifier.