Think of it as the "More" Channel....
Engaging the DRIVE 2 footswitch brings in the DRIVE 2 knob, allowing you to add distortion (in addition) to what's selected on the DRIVE 1 knob.
At the same time, the pedal jumps from the (pedal’s main, large) VOLUME knob level setting to a higher level selected on the DRIVE 2 VOLUME (mini-knob.)
1) Use as a volume Booster (same Distortion setting, only louder): Set DRIVE 2 knob to "0" and set DRIVE 2 VOLUME mini-knob to taste
2) Use as a higher Gain lead tone with Volume Boost: set DRIVE 2 knob anywhere above "0" and DRIVE 2 VOLUME mini-knob to taste. Ideally you set DRIVE 1 for your lower-distortion sounds and DRIVE 2 for the higher distortion sound…effectively giving you 2 pedals.