Please Help Me Understrand "Phase" Control

I have a "phase" button on the remote for my 2 channel integrated amp.  I have not cross wired the speakers so I assume that they are in phase with each other.  When I push the button to change "phase" on the amp, a signal light comes on the front panel so I know that it is either this or that, and I assume no light is the default position. It is on or off, not variable. There is no explanation of this function in the operator's manual. When the light is on, the sound is noticeably better;  a bit crisper both in the highs and in the sound stage separation.  So, what is happening when I change "phase" on the amp?