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Stemming and Disguising Coverage
Meanwhile, on defense, anything goes. Linebackers rush the line of scrimmage, threatening to blitz. Cornerbacks race up to bump their receivers or backpedal to give a pre-snap cushion. Linemen shift, safeties slide up into the box, and everyone points and shouts last-second instructions. The quarterback and the other offensive players must maintain discipline while reading and adjusting to the chaos unfolding on the other side of the field.
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Defensive coordinators know that pre-snap motion is their advantage, and they design all sorts of subterfuge to create mismatches, disguise weaknesses, and generally confuse the offense. While some defensive pre-snap motion is the direct result of changes in the offensive formation (like a cornerback following his receiver across the field), much of it is designed to misinform the offense about the type of coverage or the likelihood of a blitz, or to move defenders into better position to eliminate specific running or passing plays.
Coaches often call this kind of pre-snap motion "stemming." When a defense stems from one formation to another, it can trick the quarterback into making a very bad decision.
Cover-2, Man Free, Blitz or Buzz?
Let's give the offense a very easy pre-snap read, then see how the defense can stem into another look to cause confusion. Diagram 1 shows the offense in a one-back formation with a slot receiver to the weak side. The defense starts in a base 4-3 look.
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| Diagram 1 |
The pre-snap read is Cover-2. Why? First, the safeties are aligned in a classic two-deep formation: 12-to-15 yards deep, a yard or two outside the hashmarks (in college, they will be just inside the hashmarks). Second, there's the alignment of the corners, four-to-five yards deep and on the outside shoulders of the receivers. Again, this is standard positioning; the cornerback can bump his receiver and influence him to take an outside release, and he's also in good position to peek into the backfield to read the quarterback's drop or a handoff. Finally, the defense stayed in its base personnel package against a three-wideout offense, but the weakside linebacker (Will) isn't head-up on the slot receiver. He's in no position to play man-to-man defense.
Let's focus on that slot receiver. In this scenario, he has an option route assignment, meaning that he is making the same read as the quarterback on his side of the field. The slot player reads Cover-2 zone and assumes that the Will must drop 10-12 yards and widen slightly to defend against the seam route. The five-to-seven yard hitch route should be open, and a quick receiver should be able to turn upfield for more yardage before the Will can arrive to make the tackle. It's an easy read and easy yardage.
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| Diagram 2 |
Now, let's have the defense stem into a different look just before the snap. In Diagram 2, the strong safety has walked up to linebacker level, while the free safety has shifted back closer to the middle of the field. Suddenly, the Cover-2 look is gone, and there is another defender in the box.
What's the quarterback's read now? Assuming the cornerbacks stand pat, the defense appears to be in man-free coverage, with the free safety covering the deep middle. If the cornerbacks dropped to a depth of seven or more yards, the quarterback would read a Cover-3 zone, with the corners and the free safety guarding the deep part of the field. Under the circumstances, the quarterback would have to prepare for a possible zone blitz under the three-deep zone. Even in man coverage, a blitz by the strong safety or any of the linebackers is a possibility (if the Will blitzes, the free safety can cover the slot receiver).
The slot receiver, reading the change in coverage, probably won't run a short hitch route; a linebacker in man coverage would be in good position to break up the pass. His option may be to sprint up the seam to outrun the Will, or to run an out route to beat the slower defender to the sidelines. But we're making a big assumption here: that both the quarterback and the receiver read the stem properly. If one sees the strong safety creep up and the other doesn't, passer and receiver won't be "on the same page." Remember that players don't have the birds-eye view that coordinators and fans enjoy, so the receiver may not be able to clearly see the motion on the other side of the formation. He may have to read the change in coverage by watching the free safety roll to midfield or the Will widen to get head-up on the slot man.
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| Diagram 3 |
Now let's reveal the defense's actual coverage on this play: it is indeed man-free coverage, as Diagram 3 shows, but with a catch. The Will does have single coverage on the slot receiver, which is a mismatch, but he has help. The middle linebacker (Mike) will act as a "robber" on this play, lurking in the slant-to-curl zones. His job is to "buzz" routes, running underneath short throws and looking for easy interceptions. The free safety is shown covering the weak half of the field, not the deep middle, offering additional help to the overmatched linebacker (the cornerback on the opposite side of the field might creep back to increase his cushion before the snap on this call). There's no blitz, but if the quarterback anticipates a blitz because there's a safety in the box, he's likely to look left and try to hit the slot man or the left receiver on a slant or other short route. The Mike could easily pick off such a pass.
(Note: Usually, in this formation, the middle and strongside (Sam) linebackers are both assigned to read the back in the backfield. If the back releases right, the Sam covers him and the Mike buzzes the offense's left side. If the back releases left, they switch roles, and the Sam buzzes beneath the tight end and split end on the offensive right side. If the back stays in to block, both can buzz, or one can buzz while the other blitzes. And if it's a running play, two linebackers are reading the running back at the snap, which is a good thing.)
This man-free robber coverage is just one of the defense's options. The call itself isn't as important as the confusion it can cause for an inexperienced quarterback or an unprepared offense. And stemming isn't just for pass coverage. It's a great way to disrupt blocking patterns on running plays as well.
Stemming to Stop the Stretch Run
Zone blocking is all the rage in the NFL right now. A team that zone blocks well, like the Broncos, can open up huge holes in the running game and set up a lethal play-action passing attack. But zone blocking requires precision timing and quick decision-making by each offensive lineman. By stemming into a different look just before the snap, the defense can force multiple blocking adjustments and take away zone-block opportunities.
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| Diagram 4 |
Diagram 4 shows the tackle box area on a basic zone-blocking stretch run to the strong side. The offense is in the I-formation; the defense is in a vanilla 4-3 alignment. The offense hopes to string out the defensive line and create cutback lanes by executing something called a double scoop block. The diagram shows the motion of the point-of-attack defenders. All of the linemen slant to the right at the snap, creating space along the line. The center and right guard double team the 3-technique tackle, while the right tackle and the tight end double the end. Any of the four linemen can peel off his block to engage one of the linebackers on the second level, depending on how those linebackers attack the play. The fullback can stop any penetrating defender, or he can pull around the tight end and log the first linebacker who tries to turn the corner. The running back reads the chaos in front of him and picks his favorite hole. This is a difficult play to defend from this formation because the zone blocking principle ensures that there will be a blocker in every gap.
In an effort to cut off the stretch run, the defense may stem into a five-man front, as shown in Diagram 5. As shown, the Sam linebacker slides down to play face up on the tight end, while the strong safety fills the Sam position. Also, notice that the defensive line has shifted slightly. The left defensive end has moved from the tight end's left shoulder to the right tackle's right shoulder, the 3-technique tackle is now shaded to the right guard's right shoulder (the 2i-technique), and the nose tackle has moved slightly to the weak side. These minor shifts can make a big difference for offensive linemen who are hoping to stretch the defense out.
At the snap, the Sam aggressively attacks the tight end's outside shoulder. Had he done this from his old position, he would be a sitting duck for either the tight end (coming off the double team) or the fullback. But now he's attacking from the defensive line, and the tight end must block him at the snap, eliminating one double team. As an added wrinkle, the defensive coordinator orders the nose tackle to cross the center's face and attack the strongside A-gap, eliminating the center-guard double team. That's a dangerous strategy, because a great cutback runner could turn this play to the weak side; we'll assume that Clinton Portis or Warrick Dunn isn't running the ball, and that the Will linebacker is good enough to fill the backside gap.
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| Diagram 5 |
The defense effectively shuts down the stretch run with this stem look. The Sam gets too much penetration for the halfback to bounce outside, and if the offense does muster a zone double-team inside, there's an extra defender in the box to contend with. Alert offensive linemen would call off the double scoop in this scenario and do their best to drive block their defenders to create a little running room. A savvy quarterback, seeing a five-man front and eight men in the box, might audible to a play action pass. That's why defenses often stem into this look just before the snap, when it's too late for the offense to get cute.
Summary
The stem plays we just covered were just the basics. In an NFL game, it's not unusual to see three or four distinct shifts before the ball is snapped. The only limit to the amount of stemming and pre-snap motion a defense can execute is common sense. Disguising blitzes and coverages can only be taken so far. Players who will cover deep zones must be lined up somewhat deep, while blitzing defenders have to be within a zip code of the line of scrimmage. Cornerbacks must be in the vicinity of receivers; defensive tackles won't do anyone any good aligned 15 yards deep. And of course, too much motion might confuse the defense as much as the offense. But within the realm of practicality, there are an infinite amount of stems, slides, shifts, and adjustments.
So next time you see defenders hopping all over the place, feigning blitzes, and rushing into position a split second before the snap, be thankful that you aren't a quarterback. You get to watch the action and enjoy it; the quarterback (and the receivers, and the linemen) have to try to make sense of all that anarchy.
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