An expander is closely related to a gate like a limiter is related to a compressor. An expander leaves louder information (that is above the threshold) untouched while simply turning down softer information when it goes below the threshold (instead of completely turning it off like a gate). The purpose of this is to increase the dynamic range of a signal (instead of decreasing it like a compressor). An expander is not really used too often, but can be useful in similar circumstances to the gate when cutting the signal off completely is a little extreme. I used an expander once in a voice over studio where they wanted to turn down the breaths, but not completely get rid of them.