Why does my cat purr when she hunts?

I know that cats purr when they're content or frightened, but I can't find anything that explains the purpose of purring while they're about to pounce.

I have two cats but only one of them does this. When she sees something move, like my feet under the blanket or a dangling string, she'll crouch down real low and her eyes will go super wide, then she'll start purring. I've seen her do this when she spotted a bird outside the window, too.