Jeff Zgonina, the Niners’ rookie defensive line coach, was an NFL tackle for 17 years (picking up a Super Bowl ring with the 1999 Rams) before assistant DL coach stints with the Houston Texans and, last year, the New York Giants.

He shared his vision for the line with a reporter from 49ers.com yesterday, and it’s worth looking at in some detail. The article identified three pillars of Zgonina’s vision:

One-gapping Rotation Cross-training

The switch to a one-gap approach, Zgonina says, will allow players to emphasize athleticism and instinct, instead of hesitating to read the play before deciding what to do. Rotation, we’ll discuss in another article tomorrow.

The biggest news out of the article was that third pillar: cross-training. Big Z wants each player to know multiple roles on the D-Line.

"I've told all the guys, 'You've got to know at least two positions just because of the numbers on gameday.' I don't want to pigeonhole a guy at one spot."

This has several advantages. With the team’s switch to a 4-3 under front, coaches will be able to try players at different slots and see where they fit best. The extra knowledge and experience against different offensive linemen should make the DL more effective on stunts.

Cross-training also gives the team some ability to adjust fronts if an offense switches up their formation without substituting, in a no-huddle-type offense. And when injuries hit or new players come aboard (such as former Raven DE Elvis Dumervil), there is more opportunity to keep your best players on the field together by moving them around.

This also sheds some light on Arik Armstead’s work at the LEO position. Armstead is big and talented, but some analysts (notably Better Rivals podcaster Oscar Aparicio) have pointed out that he does not fit the size and athletic traits of a typical LEO.

That is less of a concern if LEO is only one of the positions Armstead plays. Maybe he’s getting more reps there because he has more to learn about the position, but will only play the position as a backup, or in certain advantageous matchups against left tackles he has an advantage over.

Still, there can be a downside to having DL play more than one position. It’s hard enough to find players with the talent to play one NFL position well. If you insist on finding people who can play multiple positions, as Chip Kelly liked to do in the backfield, you can end up with guys who are just OK — and get outplayed — at both.