By default the MAIN OUT 3.5mm output is internal routed to the EXT AUDIO input, although this can also be physically patched. This means the main output signal is fed back through to the External audio switch/dial and therefore creating a feedback loop. This can be creatively used to add distortion/overdrive to your patch, and will be indicated by the OVERDRIVE LED.
To enable this, simply switch on the External audio switch (Second switch down) and increase the external audio dial. Please be aware that the internal feedback loop will be disabled if anything is connected to the External Audio input.