If nothing is connected to the external input, the main output audio is automatically routed into this path. When the External audio source is switched on and turned up high, this creates a feedback circuit within the Model D allowing you to add distortion or thicken out your sounds even more. The External Audio in dial will control the amount of main signal fed back into the circuit. If an audio source is connected to the Ext. input, the feedback circuit will be disabled and this dial will control the amount of signal heard in your outputs.