David Sklansky's fundamental theorem of poker:
Every time you play a hand differently from the way you would have played it if you could see all your opponents' cards, they gain; and every time you play your hand the same way you would have played it if you could see all their cards, they lose. Conversely, every time opponents play their hands differently from the way they would have if they could see all your cards, you gain; and every time they play their hands the same way they would have played if they could see all your cards, you lose.
There are two ways to help your opponents play their hands differently than they would if they had complete information: 1) keep them from having complete information; and 2) make them believe that the information they do have is incomplete.
In criminal defense practice, a Nasty Little Surprise (NLS) helps with both of these goals: it keeps the adversary from having complete information in this case, and it makes the adversary doubt in the next case whether the information he has is complete.