ARC REF 3, Nagra PSA, & Phase Inversion

Last night, while I was auditioning a Nagra PSA for my primary system, at the suggestion of a friend, I inverted the phase on the REF 3. The sonic effects were remarkable. The speakers disappeared further. The sound stage became much more defined and deeper; it extended a little more forward, quite a bit more backward, and even a little wider. Imaging, which had always been quite good, was also markedly improved. And the overall musical presentation was more relaxed (for lack of a better word) and musical (to my ears). I was really shocked but also quite pleased with the changes I heard!

Can someone explain what exactly goes on with phase inversion and how it can cause the changes I heard (and perhaps other changes that occurred and I did not recognize them)?

Thanks in advance!