The gain stage is selectable, using an internal silver relay, to better match the sensitivity and impedance of the headphones being used with the Geek Pulse.
Low is best suited for high sensitivity/low impedance headphones, such as IEMs.
Mid is best suited for mid sensitivity/mid impedance headphones.
High is best suited for low sensitivity/high impedance headphones, such as large on/over ear models.
Be sure to test all modes to see which works the best for the given situation.