In main out and phones mode (switch up), the volume knob of the 896 controls the level of both headphones and main outs simultaneously. In phones only mode (switch down), the volume knob controls only the level of the headphones, with the main outs sounding at unity gain (full volume). This is ideal for applications such as running the main outs to a mixer or power amp where you would adjust the volume control. Because powered monitors don't have easy to access volume control this mode is NOT recommend for use with powered monitors. NOTE: You should be very careful about switching to phones only mode because your main out volume may be attenuated by the main out knob in main out and phones mode, and then get VERY loud when it goes to unity gain in phones only mode.