The San Francisco 49ers defense is much-improved this season and is a big reason why the team remains undefeated through five weeks of play. The biggest test of the season will be this weekend when the Niners travel south to play the defending NFC champions, the Los Angeles Rams.

The most significant offseason improvement was bolstering the unit's unremarkable pass rush, and the difference has been dramatic.

Now, after the additions of veteran Dee Ford and rookie Nick Bosa, the defensive front is causing havoc in opposing backfields, and that has an effect on everyone playing behind them. Suddenly, defensive backs don't have to cover as long, and linebackers can feed off of offensive mistakes.

"I think we know [about] the pass rushers we've added, which is a huge help to any team," head coach Kyle Shanahan said on Friday. "I think the mentality we are playing with on the D-line, just the techniques we've changed, bringing (defensive line coach Kris) Kocurek in, and playing a little different style, playing pass first, reacting to the run, being able to do both of those with just our violence and everything.

"The violence they've brought and the mentality they've brought, with the two guys added, has kind of carried over to the whole defense."

In addition to adding two dangerous pass rushers, San Francisco's decision-makers also added a dynamic defender in linebacker Kwon Alexander, who isn't afraid to put everything on the line and speed toward his target.

"Bringing Kwon in, I think he's added an attitude to the defense," Shanahan continued. "He loves football. There's an energy about him out there that everyone gravitates to."

Alexander, in his fifth NFL season, is paired with second-year standout Fred Warner to create one of the more entertaining linebacker duos to watch.

"Fred was a rookie last year who played a ton," Shanahan said, "and he was one of our leaders last year even as a rookie. Now, into his second year, he has complete command of the defense, can get people lined up, can get us into so many different things."

Then you have the improved play from the secondary, which seems to be feeding off of the pressure and energy from the defensive front and players like Alexander and Warner. The group already has five interceptions (seven total by the defense) through four games this year after hauling in just two through 16 games last season.

"We're going into our third year here with almost the same secondary," Shanahan explained. "I know this is [Richard Sherman]'s second year, Ahkello (Witherspoon) is going into his third, both of our safeties, so guys have played a lot more together. We've added more.

"I think we've gotten better, our coaches knowing each other better, adding a few things on scheme, which helps and just the mentality overall."