A poster on II explained that he expects BVG to run more of a 1 gap (attacking) 3-4 base defense than a 2 gap that Diaco had been running. TJ, agreed and said "mostly yes." So I wanted to bring some info here for everybody to see regarding the differences.
-From Wikipedia
One versus Two Gap 3–4 Systems[edit]
The 3–4 has two basic defensive variations the one-gap and the two-gap. In a two-gap system, the linemen are charged with tying up two blockers. This allows the linebackers to "flow downhill" and make tackles without shedding blocks.[16] The one gap, on the other hand, distributes the responsibility for gap coverage evenly between the linemen and linebackers. Each player had a few "key reads" after the ball is snapped. For example, the middle linebacker may be covering the strong side A gap (gap between center and strong side guard). If he sees the guard move right, then he flows with the guard. If the guard moves left, he attacks downhill and "shoots his gap." Responsibilities in the one gap vary depending on the defense.[17]
Very few teams use purely one or two gap systems in today's NFL. However, the majority of teams, such as the Green Bay Packers and the Pittsburgh Steelers primarily use the two-gap 3–4. The Houston Texans primarily use the one gap 3–4. The New York Jets use a versatile, hybrid defense combining one and two gap looks.[18]
Here's Todd Grantham talking about the difference.
Coach Grantham Explains One-Gap vs. Two-Gap in the 3-4 - YouTube
Football’s One Gap 3-4 Defense | Strong Football by CoachCP
This is from a 49er's board where, whats seems to be a knowledgeable poster explains it like this.
There's actually quite a difference. In the 2 gap or otherwise known as "double bubble", your NT is responsible for both A gaps, and needs to command the attention of the Center and Guard. Meaning you would like a mammoth NT, like a Vince Wilfork or Jamal Williams. The NT is lined up directly on the Center and is in the "0 tech" position.
In a 1 gap 3-4, which is much more common these days, your NT is in the "1 Tech" position, meaning he lines up just slightly off the center in the strong side A gap, as his role is to not really be a clogging type like in the double bubble, but a quick, penetrating Nose that's going to be disruptive and slightly tie up the RG and Center with quickness rather than brute strength to have your linebackers make plays in the backfield easier. This is why Aubrayo Franklin struggled early on with us but once we switched to a 1 Gap he started to excel.
The responsibilities of the DE's, OLB's, and ILB'S all have different jobs as well, and they are NOT the same system. They are 2 entirely different philosophies on how to control the LOS.
Read more at
1 Gap vs 2 Gap Scheme ( 3-4 defense) - 49erswebzone.com Forum