Everybody knows what a sack looks like. That's when a defensive player tackles the passer behind the line of scrimmage, often leaving a quarterback-sized imprint in the turf.

Keeping track of sacks is easy.

But the 49ers, like other NFL teams, also keep tabs on a more elusive beast known as quarterback pressures, or QPs. An assistant coach will go through each week's game film and count how many times a player forced a quarterback to move his feet or alter a throw.

There also are records for quarterback hits (QH) for every time a defender gets his hands on the passer.

Keeping track of those things gets a little gray.

For example, who leads the 49ers in QPs?

"Parys Haralson?" linebacker Manny Lawson ventured.

Nope. It's defensive end Justin Smith with 45.

"OK, that would have been my second guess," Lawson said with a laugh. "He's always around, some way, somehow."

Getting to the quarterback was a hot topic last week as the 49ers prepared to visit Green Bay. The Packers struggle to protect Aaron Rodgers, who has been sacked an NFL-worst 41 times.

Beating the Packers usually means battering Rodgers. Do the 49ers have the pass rush to get there?

Probably not, if you look at raw sack totals, where the 49ers are tied for 23rd in the NFL.

But coach Mike Singletary, defensive coordinator Greg Manusky and the players say that sacks don't tell the whole story.

That's where the QPs and QHs come into play.

Against the Chicago Bears on Nov. 12, for example, the 49ers failed to sack Jay Cutler. But Cutler threw five interceptions. The 49ers say it's because they forced him into making uncomfortable throws.

"I always say, 'Do you want interceptions? Do you want sacks? What do you really want from a defense?'" " Manusky said. "I think it is give and take. I'd like to have five sacks and five interceptions. But sometimes you just get one or the other, and that's just as good."

Ray McDonald, who leads the 49ers with three sacks, agreed.

"Turnovers are better than sacks," he said. "It can get frustrating if you don't bring a guy down, but at the same time, we know that we're doing a good job of getting to the quarterback."

Quarterback pressure stats are, like tackles, an unofficial statistic. Teams tend to lean toward the generous side when doling out credit.

Combine the 49ers' totals in QPs and QHs, for example, and it shows they have harassed the opposing quarterback 258 times out of 338 pass attempts. (Some passes can include multiple hurries).

To show just how fuzzy the math can be, the 49ers credit Smith with 45 quarterback hits, but statspass.com credits him with a combined 15 ½ unofficial "knockdowns or hurries."

Pressure, it seems, is in the eye of the beholder.

Whatever it is, the 49ers know that they need to rattle Rodgers — whether that's with a QH, QP or a good old-fashioned sack.

"As far as the fans, I think they only really understand sacks," Lawson said. "Quarterback pressures, quarterback hits, those are all keys, and that helps our defense out. If we just get a hit on the quarterback and disrupt the throw so it's an incomplete pass, it's all the same to us."

The tricky part about pressure is that Rodgers has a safety-first mentality. Part of the reason the former Cal star hits the turf so often is that he is loath to make a risky throw under duress.

Yes, he has been sacked 41 times. But he has thrown only five interceptions. The Lions sacked him five times in Week 6, but he also threw for 358 yards and two touchdowns in a 26-0 victory.

"I think my history as a quarterback has been of taking care of the football and not throwing a lot of interceptions," Rodgers said last week. "You can go back to junior college and Cal. I feel like I'm pretty smart with the ball and make good decisions."

As the 49ers studied film of Rodgers, they noticed his knack for keeping plays alive with his elusiveness. But they also thought there were times he held the ball too long.

On those occasions, the 49ers defenders know that getting to him a second too late could mean the difference between a QH or a QP — and a TD for the Packers.

"We just have to hurry up and get to him," Lawson said. "He's holding the ball for a reason. It's because a play's about to open up downfield."