Because the Bills use a high number of three-receiver sets and spread the field in an overwhelming majority of their formations, they force opposing defenses to use their nickel and dime defenses (i.e. add an extra defensive back). This helps the Bills create more space for its skill players and better matchups in the run game.
San Francisco, however, provides a unique challenge: their talent at linebacker is so excellent that they're able to give offenses different looks when they go to their nickel and dime packages. Specifically, linebackers Patrick Willis and NaVorro Bowman will be on the field together in most of their personnel groupings, even when they get to dime.
"What makes the 49ers defense somewhat unbelievable is that they can go to the nickel or dime and still stop the run fairly consistently," explains David Fucillo of NinersNation.com. "The reason for that is they are comfortable with the athleticism of Willis and Bowman. When the 49ers go to their dime defense, they often stick with two linebackers," Fucillo concludes, noting that they'll pare down to three - or even two - defensive linemen for additional defensive backs.
As for the nickel package, which Buffalo will get a lot of on Sunday, the grouping is fairly straightforward.
"When the 49ers go to their nickel, the primary difference is that nose tackle Isaac Sopoaga comes off the field," Fucillo notes. "The 49ers go with Justin Smith and Ray McDonald as the interior defensive tackles, and then Ahmad Brooks and Aldon Smith as the bookend defensive ends."
That defensive line, backed up by the two linebackers and five hard-hitting defensive backs (cornerbacks Carlos Rogers, Tarell Brown, and Chris Culliver, as well as safeties Dashon Goldson and, yes, Donte Whitner), gives the 49ers the luxurious flexibility to match up with a spread formation without sacrificing physicality against the run. This, folks, is why the 49ers have arguably the best defense in football.
Fucillo, for what it's worth, credits Whitner for San Francisco's propensity to deliver the big hit on the back end.
"(Big hits have) become part of the 49ers' identity," Fucillo says. "They are not cheap-shot artists, but rather are simply making big plays with completely legal hits. The defense had some of these big hitters before, but they have gained a certain measure of discipline this year, and I think Whitner's role as a leader of the secondary has been a key to buiding this defense."
Long story short: even when the Bills get into their usual spread this week, they're going to have a tough task in front of them moving the football thanks to the 49ers' defensive physicality out of their sub packages. Buffalo will need to perform well in their one-on-one matchups to move the ball against this excellent defense - something that they didn't do just a week ago against a so-so New England unit.