More than a few eyebrows were raised when the San Francisco 49ers announced that Eric Mangini would be the defensive coordinator on Jim Tomsula's staff.

Not because Mangini was at all underqualified or undeserving. Mangini's defensive coaching roots run deep. He won three Super Bowls with the New England Patriots as a defensive assistant and translated that success into two head-coaching opportunities (New York Jets and Cleveland Browns).

No, the initial double-take occurred because Mangini had just finished a pair of seasons as the 49ers tight ends coach, and such a move from one side of the ball to a high-ranking position on the other is rare in the NFL.

But then again, a football mind like Mangini's is probably just as uncommon. He didn't earn the nickname "Mangenius" for nothing.

When asked about his two years as an offensive assistant on Friday afternoon after the 49ers wrapped up their three-day minicamp, Mangini hinted that, at times, he almost felt like he was undercover.

For the first time in his career, the veteran defensive coach got to see the detailed inner workings of how an offensive staff prepares each week.

"It was great to sit in those meetings and hear what their problems are," Mangini said. "You realize what creates a ton of discussion. You see what's going to give them headaches when they're getting ready to gameplan."

Now back on the defensive side, Mangini said that he is a more well-rounded coach because of the experience. And what did he learn that he's now applying to the 49ers defense?

"One of the things that I believe in defensively is you have your core things that you do really well but you have to be in a position to be flexible," Mangini said. "As a play-caller, I've called games where we blitzed 30 times, and I've called games where we blitzed three times.

"I've always been a big believer to be game-plan specific. It's identifying what we can attack and what we have to stop. Then you have to have enough flexibility in the system to get those things done. So we'll evolve as the opponents go."

Something that will be a priority in Mangini's system is disguising coverages.

"If good quarterbacks know what you're in, there's always something in a pattern that can beat that coverage," Mangini said. "So I'm a big believer in making an offense make post-snap decisions, so that they have to react."

Mangini, who hasn't decided yet whether he'll stand on the sideline or in the press box during games this fall, will have to deal with the losses from last year's fifth-ranked defense, which no longer has the services of Patrick Willis, Justin Smith, Chris Borland, Chris Culliver, Perrish Cox and Ray McDonald.

The good news is that Mangini's 3-4 scheme is similar to what former defensive coordinator Vic Fangio ran. So the players who are left from last year's roster fit solidly into Mangini's mold.

"Every offseason that I've been a part of there have been huge changes," Mangini said. "Sometimes it's free agency, sometimes its retirement, sometimes it's things you can never plan for.

"But there has been really good depth built up here. If anything, the look and feel of the players is very consistent with what I've always had. (General manager) Trent (Baalke) and I were together in New York a long time ago with the Jets. So we share philosophically on what a lineman should look like, what a linebacker should look like and so on."

As for his relationship with Tomsula, Mangini, who is one of two former NFL head coaches (Tony Sparano) on the staff, said he has been a sounding board during the early portions of Tomsula's first year at the reins of the franchise.

"I try to make life as easy as possible for him so that things that were challenges to me as a head coach, I want to make sure I'm not presenting those challenges to him," Mangini said. "My main focus is to be a resource, so if he needs anything or has a question, I'm happy to. But it's also important for Jim to do things his way. That's a lesson I learned."