Forgive me if I say what you already know.
The phase switch will put the bass output 180 degrees out of time phase with the input signal, i.e. the signal likely to be coming out of your speakers, so that when the speaker cones go outwards, the sub cones will be drawn inwards.
Why do this? Depending on sub placement with respect to room bass nodes depends on the sound of bass you get at the listening position. You may find through trial and error this phase shift actually times the bass with the main speakers a little better.
For me I just can't decide and wish I didn't have the bloody option....
PS This is based on my REL Quake