The expander comes standard on the DG-20 pickguard, which I installed on my W strat. According to EMG's website, it's designed to boost the bass and treble while reducing the mids. To my ear, that's exactly what it does. It can make the bright bridge position sound louder and fuller, almost like bridge/middle when you sweep it full on. I haven't really had it that long, so I can't really tell you much more than that. Hope that helped a little.