Size Doctor Extender question

Hi, I’m new to extenders and purchased the size doctor from China. The instructions are in Korean I think. I’ve figured out how to use the device for the most part but I’m 100% stuck on one thing. How do you adjust tension?

Apparently it goes from 1000g to 2100g, but I have no idea how to control it. Does anyone know how? Or does anyone have the instructions translated to English?