Q: I opened up my pedal and see a thing that looks like it can be turned. What's that?
A: It's called a "trimmer" (or trimpot, or trimmer pot), but you can think of it as the "break my pedal" control. Turning it will break your pedal.
But seriously, it controls voltage to the tube(s). I designed the circuit to support different tubes, so I needed a way to tweak the voltage when different tubes require different voltages. Before the pedal is shipped to you, it is set at a very specific value. If you really think you want to mess with this, please reach out to me via the Contact link above and I can give you more details.