On 3rd-and-goal at the 49ers' 1-yard line earlier this season, Eagles running back Ronnie Brown took a handoff from Michael Vick, was hit behind the line of scrimmage by linebacker Parys Haralson and, inexplicably, threw the ball forward while being dragged to the turf.

Brown's bizarre fumble was immediately hailed as a nominee for the NFL's bonehead play of the year. In retrospect, though, perhaps Brown was onto something.

It has clearly become next to impossible for a back to run into the end zone against the Niners. So why not try something unusual?

Eleven games into the season, San Francisco's top-ranked run defense has yet to allow a rushing touchdown. The all-time record for fewest rushing touchdowns allowed in a season - two - is held by three teams. Since the NFL adopted a 16-game schedule in 1978, 11 teams have allowed four rushing touchdowns in a season.

The Niners have become the first team since the 1928 Providence Steam Roller to not allow a rushing touchdown in the first 11 games of a season.

The credit for a unit that leads the league in fewest rushing yards allowed per game (75.5) and is tied for second in yards per attempt (3.5) is typically given to a front seven that features three strong Pro Bowl candidates in inside linebackers Patrick Willis and NaVorro Bowman along with defensive tackle Justin Smith.

On Thursday, though, defensive coordinator Vic Fangio noted the role of the defensive backs. In last week's 16-6 loss to the Ravens, safety C.J. Spillman knifed into the backfield to dump Ravens running back Ray Rice for a 4-yard loss on a 2nd-and-goal from the 1. Three plays earlier, safety Donte Whitner had sprinted to the left sideline to knock Rice out of bounds at San Francisco's 6 after a 9-yard gain.

"If he didn't make that tackle, we don't have this discussion today," Fangio said, referring to the rushing-touchdown streak.

Whitner has a unique perspective on the Niners' run defense.

In Buffalo last year, Whitner was part of a defense that ranked 32nd in the NFL against the run while allowing 18 rushing touchdowns and nine 100-yard rushers.

The difference between a wretched run defense and one that wreaks havoc? Whitner says it's more about tenacity than talent.

"If you have guys that are just going out there and saying, 'I'm going to just do my job and that's it, I'm not going to do extra, I'm not going to help cover for the next guy,' then you're going to have holes in the defense," Whitner said. "We have a bunch of guys that when they get up there on the defensive line and the linebackers, they have guys coming to block them and they're just beating blocks, and that's how you do it. It's not about the scheme, it's about the will to beat the block and make the tackle. Do you have it or don't you?"

The 49ers clearly have it - particularly when their streak is threatened.

On 15 rushing attempts inside San Francisco's 11-yard line this season, opponents have gained 10 yards.

"It's our will," Whitner said. "It's our will to want to get the guy on the ground, to want to beat the block and to be physical."