The headphone output monitors only the signal present on outputs 1/2. Inputs 3/4 will route directly to the monitor outputs when the monitor mode button is pressed "in"but does not mirror the signal to the headphone output. The headphone output requires power to run its internal amplifier and the monitor mode feature functions whether the computer is on or off due to the fact that it routing is done passively (i.e. not using any components requiring an external power source).