Justice Scalia's illustration of the canon in his book "A Matter of Interpretation" is roughly as follows. A sign that says, "No children under 12 admitted" can be interpreted to entail that people over 12 are admitted, or at least qualify for admission on the basis of age.
A sensible rule for cases in which it works, but in general this canon is sketchy beyond belief. It's an incredibly "loose" canon, in the sense that it can be used to justify almost anything. It is not surprising, therefore, that Justice Scalia, who is generally skeptical of canons of statutory construction, likes this one. It allows him to exercise his willfulness while pretending to be faithful to the text.
To illustrate, take this text, from a sign at the entrance to the Case Law Library's computer lab:
DO NOT REMOVE CHAIRS FROM THE LABUsing the canon of Expressio Unius, we can determine that one may remove the computers from the lab; or that one can remove chairs from the other computer lab.
Most judges, of course, would reason that the authority who put up the sign could not possibly have meant these things, and would not make this determination. But to textualists like Justice Scalia, or Chief Judge Frank Easterbrook of the Seventh Circuit, intent is not a relevant consideration, unless it can be disguised as something else.
Which brings me to another point about Expressio Unius - it involves reasoning about intent. To reach a conclusion that something that is not explicit in the text should be read into the text, one invokes Expressio Unius to reason that the person producing the text surely intended it to be in the text. It's an instance of the dog-not-barking type of reasoning that judges are rightly skeptical about, and it ought to be treated as such.
Why is Expressio Unius such a loose canon? My answer is focus structure, a topic I admit I know little about. Focus is used to highlight the part of an utterance which one wishes to contrast with unexpressed alternatives. For example, if I say "Do not remove CHAIRS from the lab," I'm implicitly authorizing the removal of items that contrast with chairs, but not implying anything about the removal of chairs from other labs.
The problem with laws and focus is that they're written down. In spoken English, the way to mark focus is stress and intonation, but these things are not recorded in written English.