The mode switch on the EPX4000 (as well as the EPX2800) will choose between three different selections. The top position (MONO) is for use when you have signal plugged into INPUT 1 only. This will still carry signal out to both OUTPUTS. The middle position (STEREO) is for use when both INPUTS are being used and will carry signal out to both OUTPUTS. The lower position (BRIDGE) is used when using the power of both channels of the amplifier to supply signal to OUTPUT 1 only. This will change the wattage ratings, so consult your manual for specifications as well as the manual for your speakers before choosing this selection.