That is almost impossible to describe. The polarity would be reversed and thus off axis sound is produced. This is most commonly associated with subs ( 180 phase ) but I use my Morel Elate tweeters off axis to enhance the sound somewhat.
Most people find the sound out of sync or just "off" but some appreciate it. It is a personal preference I suppose.
I never did care for it until I really dug into it with the Elates. It is not something that I would recommend though. It just something people can experiment with as everyone hears differently...