The phase control determines whether the subwoofer piston-like action moves in and out in phase with your main speaker or opposite the main speaker.Proper phase adjustment depends on several variables such as subwoofer placement and listening position. Ajust the phase switch to maximize bass output at the listening position.There are no right or wrong setting ,this switch offers the added flexibility to ajust your sub for optimum performance for your specific listening condition without having to move your other speakers.You should experiment with the phase switch in both position and leave it in the position that maximizes bass performance.Pat.P