Dunlop Uni-Vibe trimpot inside ?

[RV-music91]

When I installed a new battery inside my Dunlop Uni-Vibe stereo chorus , I noticed a tiny hole in the pcb wich gave acces to a little white trimpot.

Does anyone know what this trimpot does ?

I'm a little scared to turn it because I don't want to ruin my pedal or anthing....