Native village is an example.
Suppose you have bought exactly one scrying pool and one university; you know that both of them are on your Native Village mat. Your opponent does not know that they are on the NV mat; they may know that you haven't played them, but since there's been a lot of Minions going around, that isn't unusual. You have lots of stuff in your discard pile, and your deck is empty.
In your hand, you have four coppers and a Native Village. This is a very GOOD hand, since you can play University to gain an inn and shuffle many actions from your huge discard pile into your empty deck, and then play the Pool to draw them.
If your opponent does not look at your hand, they don't have much information about your possible turn at all; they may correctly play it safe, as if you have a good hand (not break PPR, or equivalent.)
However, if they play a bureaucrat and look at your hand, they will see that you have a native village, with only two cards on the NV mat, and four coppers. This is EXTREMELY likely to be a bad hand - for most possible contents of the NV mat, given all the things that COULD be on there, you cannot have a very good turn. The opponent will then play as if you have a bad hand (since, as far as they know, you do.) This will be a mistake, since you actually have an amazing hand and will end the game on a win.