Focus vs. exposition

Here's some advice for suspense and mystery writers from Writer's Digest: 

“Maintain focus.  Don’t interrupt the scene with exposition.”

Well, now consider Tea Obreht's "The Laugh."  (and if you haven't read it, see http://www.theatlantic.com/magazine/archive/2009/08/the-laugh/7531/ )  Tea does a swell job of building tension and momentum.  Then she breaks off and gives you something that happened in the past.  A flash back, an exposition, call it what you will.  I think it works because she gets us to read the exposition by teasing us with the promise of what's going on in real time.  All we gots to do is get through this little piece of background, and we can get back to the action. 

What do you think?