Why do many grazing herbivores have a white rump patch?

I have noticed that many species of deer and gazelle have a white rump. I think it is widely believed that this acts as a visual signal for the rest of the herd which tells them when danger is close, but if the white patch is always on display (it can't be covered up) then how can this be true? Wouldn't they always be able to see it? So in what way does it tell them danger is approaching?