In the words of Old Blood Noise Endeavors:

The Old Blood Noise Endeavors Expression Slider is a fader for use with any pedal that has an expression input. Simply use a TRS cable to plug the Expression Slider into the Expression input and use the slider as you would a knob or expression pedal. Suggested uses: strap it to your mic stand to adjust your favorite setting without bending down, keep it at your studio console to control a parameter while tracking, use the indicators to mark your favorite settings, or simply replace a knob control with a slider because let’s face it: sliders are cool.