Rookie DL coach Jeff Zgonina detailed his vision for the 49ers defensive line in an article on the team’s website Wednesday. He listed three pillars to his scheme:

One-gapping Cross-training Rotation

Yesterday, we discussed the cross-training pillar. Today, a closer look at the rotation element.

The Niners have invested heavily in their front line, spending their 2015 and 2016 first round picks on DL Arik Armstead (17th overall) and DeForest Buckner (7th), without much to show for it.

Last year, the team was dead last in rushing yardage allowed, and it wasn’t even close. Only five teams allowed more than 122 yards per game; the Niners gave up 165.9, fully 23 yards higher than second-worst Cleveland. Injuries in the middle, to nose tackle Ian Williams and both inside linebackers — Ray-Ray Amstrong and Navorro Bowman — didn’t help.

So this off-season new GM John Lynch doubled down, taking Stanford’s Solomon Thomas with the 3rd overall pick and signing two notable free agents, NT Earl Mitchell and DE/OLB Elvis Dumervil.

Going into mandatory minicamp, all of these acquisitions have left the team with a deep roster up front, for once. Discussion has shifted to happier topics such as how to get everyone on the field, or which name players get cut.

That depth fits perfectly with Zgonina’s emphasis on rotation.

“I’m a rotation guy,” Zgonina said. “I’m allowed to rotate guys as much as I want, and I will do that. I told those guys, ‘You give me four-to-five plays full-tilt, I’m going to rotate you.’ I believe in fresh bodies all the time.”

It’s hard to argue that point with a guy who lasted 17 years in the league and played until he was 39. In 2009, his final season, the only non-kicker older than Zgonina was Brett Favre. That’s not the most striking detail, either. According to an amazing portrait of him that year in Sports Illustrated, he missed only one game in 17 years due to injury (a broken finger). And never made more than the league minimum.

Coach Zgonina knows DL rotation as well as anyone on earth, from his own experience. He began every game on the sideline until his 9th season, when he started 11 times for the Super Bowl champion Rams. But he had more sacks (4.5) the year before, coming off the bench on a team that went all the way.

Last year, rookie DeForest Buckner was the team’s best lineman with 73 tackles, six sacks and two fumble recoveries. Despite missing a game with a foot injury, he also played more than 1,000 snaps on defense — and another 103 on special teams.

Incredibly, he had 11 tackles and two sacks as late as Week 14 against the Jets, though he faded a bit after that. But the Niners may have dodged an injury bullet working him that hard. Without naming any names, Zgonina agreed.

“I don’t like to see guys play more than 1,000 snaps in a season.”

No matter how much he rotates, Zgonina won’t be able to keep all 13 DL currently on the roster. The 49ers have carried seven defensive linemen on the roster recently, though the switch to a 4-3 front makes an increase to eight likely.

Matt Barrows had an excellent breakdown of all 13 and their prospects for sticking with the team Tuesday. In his take, Tank Carradine is finally playing his natural position and is in good shape, but other returning veterans such as Aaron Lynch and Quinton Dial may not survive.

While they’re new to the team, players such as Chris Jones and Earl Mitchell played for Coach Z in Houston and exhibit the traits he loves: hustle, hard work, running to the ball. That will appeal to the guy who was described this way by his former teammate and former coach, Pro Bowl LB Kevin Greene:

"Dude is not physically talented, but he's got a big heart, a big work ethic and a huge presence in the locker room."

Zgonina is a first year DL coach now, after a year each as assistant DL coach for the New York Giants and Houston Texans (with a couple years of competitive dog showing in between).

He’ll work closely with veteran linebackers coach Johnny Holland, who was LB coach in Houston from 2006 to 2010 when both Zgonina and Niners’ defensive quality control coach Demeco Ryans played in the Texans’ front seven.

That history should help the new coaching staff work together. And even last year, the defensive line showed some playmaking ability. Team sack totals weren’t nearly as bad as rushing yardage; San Francisco was in a 5-way tie for 19th with 33, thanks to Buckner and Ahmad Brooks with 6.0 each. And the team tied for 9th in rushing fumbles generated with eight on the season, keeping half of those.

There’s a lot of room for improvement on this defensive line, and a lot of work to be done. It may take a couple of years for it to come together, with all the changes in roster and staff. But you’d be a fool to bet against Jeff Zgonina’s grit or work ethic, and this unit has the potential to become truly dominating.