The inputs of this subwoofer take the full-range signal from your mixer.  This signal then goes to the Thru ports and Outputs.  The Thru connections pass the full-range signal to any other device that can accept line-level signal.  The Outputs of the B1800D-Pro use the internal crossover of the subwoofer to send the mid-range and high-frequency signals to your main speakers, while the lows remain at the subwoofer