Just what the title says. The expression "pedal" is not the typical one - see the pics. It's a potentiometer that can be turned with the foot. I can't find the model on line any longer, but it's great for setting up as a volume pedal or boost at the end of the effects chain to control the overall volume of the unit with a nudge of the toe. Really helped to "tame" the whole machine for me. Don't recall the manufacturer but it was $30 on ebay. Also includes an M-audio MIDI->USB cable which allows for firmware updates and backup. It's this one, $40 on MF. Unit has some expected play wear, minor scuffs and such, but is in great shape with no issues. There is velcro on one side of the expression knob. Ships in the original box. $290 shipped in CONUS. Seems like a deal, $50 less than a new one and with $70 in added extras. I love this unit but my needs have changed - gotta have amp emulation for a new project. Trade for a Zoom B3 plus a little cash from you.