Bob, the previous replies have covered it pretty well. There's nothing complicated about the "phantom" effect; whenever two speakers play the same sound at the same loudness, it appears to come from midway between them(for someone sitting in the middle), as if a speaker was there. This can be heard whenever you listen to a stereo program on two speakers and something is playing that's supposed to be in the middle(e.g., often a singer, or an announcer on an FM radio program).
As mentioned, it has pluses and minuses besides saving money on a separate center speaker. The main speakers may be of higher quality than the separate center speaker would be. The center image will move to the left or right when the listener isn't in the middle, but not farther out than the speaker on that side. Also, as Dean pointed out, with a separate center speaker the level of sound that's supposed to come from the middle(e.g., most movie dialog)can be independently adjusted to make it a bit louder and easier to understand, which of course can't be done when the main speakers are handling everything.
Trying this is free, of course, and setting your HT receiver to show that the center speaker is "none" results in all center sound, even from multi-channel discs, being sent to the main speakers.
