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For the first time in three years, there is scheme continuity on both sides of the ball. This is important for player development and team success as returning players don't have to pick up a new playbook and can build upon the foundation from the previous year.
Last season saw the return of the classic, yet modernized, West Coast Offense. Kyle Shanahan installed many elements that brought back nostalgia from the 49ers Championship days. A running back led the team in catches, a fullback was lead blocking and was a part of the passing game. A myriad of formations, presnap motions, and personnel groupings were used to run timeless WCO concepts, the passing attack stretched the field horizontally just as much as it did vertically, isolating 1v1 matchups in the passing game to get playmakers in space, and quarterback bootlegs that fed off the running game.
In 2017, with OLmen known more for their power, Kyle ran many inside zone concepts to fit the scheme to the players. This offseason, there was a concerted effort to reinvent the OL to better execute the outside zone run - the run concept Kyle wants to master as a team. Weston Richburg, known for his agility, was signed to anchor the middle. Mike McGlinchey was drafted to be an athletic run blocker that could better execute on outside zone. The RG position was addressed by having Joshua Garnett lose weight to become more agile, and Mike Person was brought in, a longtime vet that graded highly on his athletic scores. On a whole, the OL is better equipped to run outside zone. How they come together as a cohesive unit will determine whether the run game improves.
Successful outside zone runs will get the defense flowing left/right.

Once the offense can threaten with outside zone, QB bootlegs are more effective and LBs can be attacked in the passing game.

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2017 marked the return of a base 4-3 one gap defense, backed by a Cover3 scheme. This was also a flashback to the Championship 49ers when the defense was led by George Seifert. As the defensive roster gets molded to fit this scheme, the returning young defensive players will benefit greatly from playing in the same defensive scheme, a scheme that prides itself on letting the players play fast and instinctively - "all gas, no brakes" - which is helped by familiarity. Promising youngsters such as Foster, Colbert, Thomas, and Witherspoon will be counted on if the defense is to take the next step.
Much has been discussed about the pass rush and its lack of explosive talent among the Edge. The team employed Chris Kiffin as a "pass rush specialist" to coach up the pass rush both at an individual player level and on a team level.
Kiffin: "A big thing I saw was guys not necessarily rushing together," Kiffin said of last year's (2017) squad. "You can't look at it as four individual rushers. It's really, you're rushing as one. If we can get that fixed, the pass rush improves."
Saleh: "Creating a black and white picture so those guys understand exactly what job each person has, so they can learn how to play off of each other and they can rush as a unit."
"Rushing as a unit" and "scheming pressure" are terms that will be heard throughout the season. Any deficiencies from the edge rushers can be masked if the team can execute timely blitzes that break down the opposing offensive line's protection call. Using creative blitz packages and line stunts, the team has to find ways to manufacture QB pressure. The improved CB unit could play a big factor in the overall development of the passing defense.
Giving various presnap looks in an attempt to confuse the QB/OL will go a long way in succeeding with blitzes. On this play, the 49ers overload the left side of the OL.

Once the ball is snapped, the slot CB blitzes and it's actually an overload blitz to the OL's right side, creating a 3v2 situation.

Quick QB pressure can lead to bad throws, which achieves the goal of making the QB uncomfortable in the pocket.

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DLine stunts and games can be classified under "rushing as a unit". On any particular stunt, some rushers' assignments are to occupy a specific OLman and open up a rush lane, so that their teammate can apply the pressure. "Scheming pressure" is to understand the OL protection call, and exploit its weakness.

The looping LB moves the QB off his spot and gets the job done.

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Many of the terms used in this film thread are described in the thread linked below, and all over the web.
Coverages and Concepts