As everybody knows, the Giants' defense is on a roll right now, not having given up an offensive touchdown in 10 quarters. Many, particularly those who have been incessantly beating the drum for upgrades to the Giants' linebacking group have pointed to the acquisition of Jon Beason as one of the key factors in this defensive resurgence. In fact, however, Beason is only a very small part of the recent defensive success.
Beason is a good player, a canny acquisition, and his play has definitely been an improvement over the overmatched Mark Herzlich. But in reality Beason's play has been about average in the games he's played (I didn't see the Bears game, but in the Minnesota and Philadelphia games he appeared solid, but not stellar). His PFF scores show that other observers have not seen anything outstanding in his play.
So what is behind the defensive renaissance? In a word: offense, both the Giants' and the other teams'. In the Philadelphia and Minnesota games the Giants' offense kept turnovers to a minimum and mounted several sustained drives. The defense was on the field for substantially shorter periods of time and was not constantly faced with short fields. Also, in both of those games the Giants were playing with a lead, which gives the defense the freedom to be a bit more aggressive. While the offense still has a myriad of problems (penalties and red zone inefficiency) the simple fact of reducing turnovers is the single biggest factor between the Giants being 4-4 or better at the midpoint of the season and the actual 2-6.
On the other side of the ball, Minnesota and Philadelphia were playing with substandard quarterbacks allowing the Giants to focus on their strength, stopping the running game. Without the fear of a crisp mid range or long passing game, the Giants were able to sell out to stop McCoy and Peterson. The real test will come after the Oakland game, when the Giants have to play teams that do have good quarterbacks and passing games.
Other factors are in play too. The rapid development of Will Hill has been terrific, as has Jacquain Williams' coverage game, and some decent play from Trumaine McBride. Improvements in the pass rush and more effective blitzes have all helped too . And yes, Beason has helped too. Overall I believe the defense has been average all year and that it's average now. If the Giants are going to have a successful second half, however, it's the offense that will have to pick up its game: less penalties, more touchdowns, less three and outs, and low turnovers.