Actually I'm not too sure about the total definition of this type of Buddha, as I am not the type of person who has a lot of interest in theory. However, from my understanding, silent Buddhas actually aren't totally silent, I think they only teach Morality, they do not teach the higher stages of the practice. I'm not too sure about that, but that's what I think this means. So they're not totally silent in the sense that they don't teach at all. They just teach Morality, but not the higher stages of the practice that can get people Enlightened. I'm not too sure why that is, but that is what I have read and been told.