well what i figure it does, is make the aggressor, more aggressive against his image, rather than the newly added fish.
I think of it this way, if you have an established yellow tang in the tank, and add another yellow tang and a blue hippo at the same time, the yellow is more likely to go after the newly added yellow tang, rather than the blue hippo. They are more aggressive to something that looks similar to them in appearance.
It has worked for me in the past, i would leave there a few hours a day, especially around feeding time. After a while he would forget the newly added fish was even there.
GL, HTH