You dare - daring is what all magick is about. You know - because you have to know what you are doing and what you are doing it about. You will, because your will is your primary source of strength. And so we come to being silent. You are silent because that is another source of your strength.
There can be (and there indeed are) times to speak of what you are doing, but most often, silence serves best. It's that way with most endeavors. In the office, at a hobby club, in a church (coven, mosque, or whatever) or most anywhere else, it is so often true that the one who speaks the most is not the one who knows the most. In fact, often the one who actually does know the most is the quietest of all!
There are many reasons for keeping silent. If the person you are doing the magick about is someone who caused harm, their reaction is not going to be supportive of your work - to say the least. And besides, those who need to atone need to see themselves for themselves.
And even if the person is wanting what you are offering, it can often help to not have their emotions get in the way of your work.
When it works, doubters doubt their doubts. It happened here recently, and we'll get into that in a later episode of "In A (very different) Pastor's Study", the show that's taking the place of "The sabbat School Class".