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The 49ers Believe In The Magic Of Pre-Snap Motion

Anyone who follows football and can fog a mirror knows that winning the turnover battle is important. Games often hinge on a team getting an extra possession, and when analysts have tried to quantify the edge, they’ve found that it’s large: Teams win around 78 percent of the games in which they have a positive turnover differential.

What’s less well-known is the size of the edge that play-action and pre-snap motion provides.1 It turns out it’s also considerable: Since 2017, if all you know after a game ends is each team’s share of offensive plays that featured play-action and the share of plays that had some form of pre-snap motion, you can correctly guess the winner around 65 percent of the time.2 This is fairly impressive, since neither metric directly measures the volume of yards gained or points scored. Instead, these are play design choices made before the ball is snapped and the play’s outcome is determined. And, unlike turnovers, both pre-snap motion and play-action rate are completely under a coach’s control.

While the league has been slow to aggressively exploit these edges, it’s clear they aren’t unknown to San Francisco 49ers head coach Kyle Shanahan. Shanahan’s offenses have featured large helpings of pre-snap motion and play-action fakes each year of his tenure with the Niners. Since taking over in 2017, Shanahan’s teams have led the league every year in pre-snap motion rate —  a rate that’s grown each season he’s been in San Francisco. During that same span, the 49ers have hovered around 10th in play-action percentage, peaking at second in fake handoffs in 2019, the season they lost to Kansas City in the Super Bowl.

As we enter a new season, these rates are ones we should expect to continue. The year-to-year correlation for play-action rate is robust for a football stat (0.62), and that correlation is even higher for motion (0.75).3 And if Week 1 is any indication, Shanahan intends to take things to an entirely new level in 2021. On Sunday versus the Detroit Lions, the 49ers registered their highest rate of pre-snap motion in a single game since Shanahan became head coach and notched the second highest share of play-action in a game during his tenure, running fakes on over half of the Niners’ offensive plays. Both marks were far and away the highest of the week in the NFL, with the play-action rate (51.9 percent of plays) more than double the league average (24) and the motion rate (87.3) nearly double the league average (45.3). 


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Yet what elevates Shanahan’s play design is that he isn’t just concerned with the deception offered by play-action or the hesitation that motion can create in defenders. Instead, he integrates the edges they provide into a cohesive whole that completely disguises the offense's intent.

Two plays from Sunday illustrate Shanahan’s ability to design plays that look nearly identical. Called back-to-back in the red zone, both plays feature the same formation, the same pre-snap motion, the same offensive line movement, a similar path for the running back and identical bootleg action from quarterback Jimmy Garoppolo.  

Defenders who were forced to flow to their left to rally and tackle rookie running back Elijah Mitchell in the first play are caught off guard when, on the following play, Garoppolo fakes the handoff and throws to the flat on the opposite side of the field for a first down. Shanahan designs and calls play sequences like this multiple times a game.

Not everything is roses in the city by the bay, though. San Francisco nearly let the Lions game slip away from them in the last three minutes, ceding 16 unanswered points. And the win was a costly one: The Niners lost Jason Verrett, their best cornerback, as well as starting running back Raheem Mostert for the year. The entire NFC West is undefeated through one week of play, and our NFL Elo ratings have the Niners ranked third in the division based on overall team strength. 

Still, so long as Shanahan continues calling innovative and effective plays on offense, it’s not hard to imagine San Francisco faking out the league once again and rallying for a playoff berth.

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Footnotes

  1. Pre-snap motion is any movement before the snap of the football by a player behind the line of scrimmage. We’re including all motion at the snap and before it, even if the player then became set at the time of the snap.

  2. Based on a binomial regression analysis on 2,080 team performances from 2017 through Week 1 of 2021 with the sample split 80/20 into training and testing. Losses in out-of-sample data were predicted correctly 146 times with 77 false negatives. Wins were predicted correctly 126 times, with 67 false positives. It’s important to note this is not the same as predicting outcomes pre-game, but rather an indication of the impact play-action and pre-snap motion have upon winning.

  3. Based on team season pairs from 2017 — the first year for which we have data — to 2020.

Josh Hermsmeyer is a football writer and analyst.

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