http://www.baltimorebeatdown.com/2012/9/14/3332258/ravens-defense-bend-but-dont-break
However, when you look inside the numbers, it's not the yardage allowed that will impress you as much as the points allowed and turnovers. Traditionally, the Ravens mantra on defense has been "bend-but-don't-break," meaning they will lay off opposing offenses between the 20-yard lines, but stiffen up once the opponents gets into the Red Zone.
http://football.calsci.com/Positions7.html
There are a few philosophies on how to operate a defense. Two of the most popular are called "attacking" and "bend but don't break." In an attacking defense, the people on the defense will be constantly blocking and jamming receivers, and they will use many sneaky methods to try to get into the backfield and sack the quarterback. The favorite sneaky method is called a blitz, which is where you have one, two, or three linebackers, cornerbacks, or safeties abandon their normal post and rush in on the quarterback instead. This is a big gamble - if it pays off, the defense gets a big win. But a very good quarterback can often see this coming and throw the ball to a receiver who has been abandoned, which usually results in a play of 20 yards or more.
In the bend but don't break philosophy, the defense will pull back a bit and do everything they can to insure that the offense never moves the ball more than four yards or so on a given play. The idea here is that at four yards per play, if the offense has to move the ball 80 yards so score, they have to have 20 good plays pretty much in a row. It's believed that very few offenses can put together 20 good plays without making a mistake. The bend but don't break defense will be watching very carefully for these mistakes, such as a fumble or a poorly thrown ball that can be intercepted. It's important in this defense that the defensive guys themselves make very few mistakes.