Depending on where the subwoofer is placed in your room, there may be bass frequency cancellation. This can occur because your subwoofer and front speakers are out-of-phase—they work against each other through their frequency overlap region. Bass is then reduced and may even sound disjointed. This control accurately synchronizes your subwoofer and front speakers through their bass frequency overlap region.

Subwoofers either come with a variable control knob allowing finer adjustment or a switch allowing you to choose between 0 and 180.  Every setup will be different, but the general room of thumb is to set the subwoofer at 0 if it is placed at the front of the room or 180 if it is in the rear and adjust from there.  It’s easiest to set with 2 people, have one sit in the main listening position while playing bass heavy content and adjust the knob or switch until bass sounds the fullest.