This is the first part in a two-part Niner Noise series looking at new San Francisco 49ers defensive coordinator Jim O’Neil’s defensive scheme.

In late January, the San Francisco 49ers hired former Cleveland Browns defensive coordinator Jim O’Neil. It wasn’t the kind of hire that made any among the fan base particularly excited to hear.

Senior Producer for NFL Films and Yahoo!’s Shutdown Corner contributor Greg Cosell had this to say about the young defensive coordinator after the hire:

I think he’s really sharp. I think he’s really passionate. I think he really cares. Obviously people are going to look at Cleveland’s defense which was not very good this year, and it was not very good for a number of reasons. But I think he’s a bright young coach. Now, don’t forget, when you’re young too as a coach, you go through a learning process as well. That’s part of the gig. People just automatically assume because these guys are coordinators or whatever, that they know everything. You go through a learning curve in anything you do when you’re fairly new at it, and I think Jim probably went through that in Cleveland. They had some struggles on the defensive side of the ball. They didn’t have a ton of talent, but they certainly did have some struggles.

Cleveland’s struggles were due to a number of factors, not least of which were key injuries to cornerbacks Joe Haden (concussion), Justin Gilbert (concussion), defensive end Desmond Bryant, safety Tashaun Gipson (ankle), and linebackers Craig Robertson (ankle) and Scott Solomon (ankle). Cleveland ranked 23rd overall in defensive efficiency (27th against the pass, 26th against the run), according to Football Outsiders metric, DVOA.

The Browns’ struggles were highlighted midseason that causes some alarm among players and fans:

Recently, a number of players have told me what they believe is the main impetus behind the continual running-back beatings: The Browns have issues with gap integrity—football speak for jamming the holes between the offensive linemen. Rather than being assigned specific gaps, Cleveland’s defensive linemen play different techniques based on how their offensive counterparts are blocking them. The linebackers, then, are expected to guess what technique their teammates are using, scrape through the resulting mess and make the play. Opposing offenses have identified this flaw on film and are repeatedly, week-after-week, gashing the edge of Cleveland’s defense. It’s a completely chaotic approach to stopping the run, and players have said—off the record—they’re spending way too much time thinking, and not nearly enough time reacting. “It’s an entire guessing game,” said one source. “Imagine trying to define mud.”

This critique and their on-field performance and statistics don’t exactly inspire confidence among fans as San Francisco was only marginally better under former defensive coordinator Eric Mangini. Add this to the fact that fans are concerned that Chip Kelly’s offense will be a detriment to the defense based on how many plays are run and how long the defense would be on the field.

But for Chip Kelly’s defenses in Philadelphia, as I detailed here, the numbers don’t support the idea that his offense puts the defense at a disadvantage. Philadelphia was 15th overall (25th against the pass, 11th against the run), in 2014 they were 10th (18th against the pass, 7th against the run), and in 2015, they were 26th (14th against the pass/28th against the run).

In 2013 and 2015, Philadelphia led the league in opponent plays per game and in 2014, they were second in opponent plays per game. In each season the Eagles were just 5-7 plays above the league average and compared to the top 10 teams each season, the margin was even narrower, with teams such as the Carolina Panthers and New England Patriots sharing the top 10. This is an indication that plays per game as negative idea does not stand up to scrutiny.

So who is Jim O’Neil and what can fans expect as far as his defensive philosophy? O’Neil is a disciple of the Rex Ryan/Mike Pettine school of defense. O’Neil began his NFL coaching career with the New York Jets under Rex Ryan as the defensive quality control and defensive backs coach, moved to Buffalo with Mike Pettine and worked as the linebackers coach, and moved again to Cleveland in 2014 with head coach Mike Pettine and worked as the defensive coordinator for the 2014 and 2015 seasons, though it is often thought that Coach Pettine really ran the defense in 2014.

O’Neil runs mainly a hybrid 4-3 defense with some 3-4 principles sprinkled in. This post will cover O’Neil’s base and hybrid defenses against the run and future post will cover the passing defense coverages.

The Browns defensive scheme was “multiple” especially in sub packages (3-3-5/4-2-5 nickel defenses), and its base defense truly blurred the line between “4-3” and “3-4.” This “hybrid” defense allows the defensive line to 1-gap and 2-gap their opponents depending on the flow of the play. About 50 percent of the time, the Browns defensive front played with three or four down-lineman.

To understand what I mean by a “hybrid” front between 1-gap and 2-gap technique, you must first understand what 1-gap and 2-gap technique is. 1-gap is easy. Every defender in the box is responsible for a “1-gap” and fills that gap on a run play to tackle a hypothetical ball-carrier. Against the pass, everyone who is designated to rush the passer will do so through his designated gap.

In a 2-gap defense, the defensive lineman have to read and react to the play. On the run, the play-side determines which gap the linemen will crash. 2-gap linemen are also trying to soak up blockers to free up the linebackers to make plays. Here is an example of how a defense might two-gap from Nick Saban’s Alabama playbook:

In Saban’s defense, all linemen are responsible for one of two gaps. The ends are responsible for the B-gap when the play is away from them or the C-gap when the play is toward them. The nose tackle will usually rush the opposite gap of the playside in order to soak up the blockers and let the linebackers free flow to the ball carrier.

The hybrid base in the Rex Ryan/Mike Pettine/Jim O’Neil relies on 1-gap principles on one side, and 2-gap principles on the other. Below, you’ll see nose tackle aligned in a 1-technique, while defensive end between he tackle and tight end is in “heavy 5” technique. These are the two-gapping linemen. On the opposite side of center, you’ll see defensive tackle in a 3-technique with the hybrid defensive lineman/linebacker in a “ghost 9” technique, outside of an imaginary (“ghost”) TE. Those are the 1-gapping linemen.

(Image: NFL GameRewind)

The playside goes to the Jets left. The 1-gapping linemen get into their gaps fast and together with the 2-gapping side, soak up the blockers allowing the linebackers to flow downhill, stopping the back for a one-yard gain. In the gif below, pay particular attention to the gaps the linemen fill and how the linebackers react.

(Images and Gif: NFL GameRewind)

The 49ers have the versatility, and like them to add another defensive linemen in the draft who adds to the core of the defensive front. I expect the 49ers to play more 3-4 base fronts but the same principles still apply as they loaded up versatile defensive lineman in the last several drafts. Up next: the passing defense and coverages.

All statistics, records and accolades courtesy of Pro-Football-Reference.com and Sports-Reference.com unless otherwise indicated.