This pedal is nothing more that a variable resistor that can be bypassed via the foot-switch.
The power supply is just for powering the ON/OFF LED indicator - the pedal works without power. However, that does annoy me a little bit because there are mechanical solutions that would do this job, even in the dark. I get that an LED circuit is probably cheaper to make if you're buying those parts in bulk anyway, but it still greaves me a little that EH chose to do this. The reason I got this pedal is because I'm front loading a 5151 overdrive with a Unit67 boost, and wanted to be able to replicate the overdrive's output attenuation when it was off. Now I have the overdrive's output attenuation at unity gain and this Signal Pad pedal does the real attenuation. This allows me to keep the boost engaged when the overdrive is off without overloading amp simulator I have later in the signal chain.
It's a pretty inexpensive addition to my pedal with lots of tonal benefit.