Why have we seen/heard so little of the deep pass during the preseason? The answer lies partly in the injury to Denarius Moore (which reduced the number of deep-threat receivers), but is fundamentally rooted in the zone blocking scheme of the rushing offense that the Tennessee Volunteers now employ.
The Cost of Zone Blocking
The basic principle of a zone blocking rushing attack (outlined in great detail here) is that the offensive linemen block out regions rather than players. If their designated region is vacant of defenders, the linemen then move along a progression of regions to the next block assignment. (That's a bit oversimplified, but accurate to the general concept.) The key behind this offensive philosophy is that the offensive linemen must get to their positions before the defenders so that they can have the advantage of leverage. Speed is favored over power in a zone blocking scheme, as is discipline over brute force.
The emphasis on speed over power usually requires that a sacrifice be made; offensive linemen that are both quick and huge beasts are extremely rare, so the linemen in a zone blocking system usually end up being a little smaller and lighter than their counterparts in a man blocking system. Unfortunately, this size reduction carries over to the pass-blocking game, and the cost-benefit exchange is not as favorable.
At the Mercy of the Defense
Football rules state that seven offensive players must be lined up on the line of scrimmage, five of whom are ineligible to receive passes. "On the line of scrimmage" typically means that their shoulders are lined up within a yard of the line, or whatever the officials' judgment dictates. "Ineligible" means that they cannot receive passes and they cannot advance beyond the line of scrimmage prior to a pass (unless the pass is received behind the line of scrimmage). So, assuming that we are talking about passes beyond the line of scrimmage, the linemen (who are at the line of scrimmage) can only move backwards or sideways at the snap of the ball. They are then at the mercy of the rush of the defensive line to determine exactly how they block.
Here, the size limitation becomes a disadvantage. Larger offensive linemen have extra weight that must be negotiated for a rusher to reach the quarterback – particularly in the middle of the offensive line. Zone blockers are at greater risk of simply being pushed back into the quarterback because their lack of weight and their inability to dictate leverage give the defender the advantage.
In short, the time window for passing tends to be smaller for a zone-block team than a man-block team. The passing offense must compensate for the time deficiency through other means.
As the time to pass decreases, the time to make a decision must also decrease. Either the quarterback must be able to make snap decisions as the play is in progress and run through the reads quickly, or the decisions must be made a priori – before the ball is even snapped. Consider Denver: when Shanahan brought in the zone blocking system, he had John Elway – a veteran who had made a career out of decision-making under pressure (just ask Cleveland – twice). Between the rush-blocking and the quarterback capable of compensating in the passing game, the Denver rush thrived to the tune of a 2,000 yard season for, and the beginnings of the 'plug-n-play' running back era. After Elway came Brian Griese, whose decisions were not as fast: the offense immediately suffered.
If the fly-by-wire decision making is not sufficient, then the decisions must be simplified. This is where Kiffin's offense is said to be 'quarterback-friendly'. As the team lines up, the quarterback reads the safeties and picks the appropriate half of the field, reducing the decisions in half. After pre-snap adjustments, the quarterback then chooses his #1 and #2 targets. Suddenly, the 5 eligible receivers are reduced to 2 (at most). If neither option becomes available during the play, the quarterback is supposed to throw the ball away and worry about the next play. Without digressing into another Clawson-bashing system, compare this to the 2008 Vol offense, where each receiver had up to 3 different pre-snap reads to make, and the quarterback had to understand and correctly identify each pre-snap read. (Independently, such a system can have 15 different route possibilities - 3 each for 5 receivers; these possibilities result in as many as 35 = 243 different route combinations on a single play.)
Back to the point: the quarterback must be decisive. One of the costs for a zone-blocking scheme in the rushing game is that the quarterbacks must be willing to give up on a pass play – to become game managers. (Look at Elway's last two years; for all his talent, he was largely a game manager during both Super Bowl runs. A really, really good one who could still shred a defense, but a manager nonetheless.)
Shorten the Passes
It's simple, really; throwing deep requires giving enough time for receivers to run deep. If you limit the deep routes, the quarterback is not asked to hold the ball as long. You still have to run enough deep routes (and occasionally pass to them) to keep the defense honest, but the focus shifts more to the short-to-intermediate passing game. This is why so many zone-blocking teams adopt West-Coastish passing systems; the routes are based on timing and precision more than stretching the field. It doesn't have to be complex, but the routes must give the quarterback a chance to throw early if he is to throw at all.
Two favorite Kiffin/USC tricks highlight this point. One is a route where the wide receiver begins by running straight down the sideline, then turns back to the line of scrimmage as the cornerback turns. Per Gate21:
Gerald Jones played today, and had a couple of nice catches. Quintin Hancock caught a touchdown after catching a short pass and breaking a tackle. The Vols also ran this route a number of times: with one-on-one coverage on the outside, the receiver runs a go route up the sideline, and the QB throws it on a line to the wideouts back shoulder, catching the cornerback with his back turned to the play. They tried this play three or four times, completing one of them.
The pass is then made such that the receiver is actually running back for the ball. This route sacrifices distance for safety; it's a quick pass with a very low probability of a turnover, especially against man coverage.
The second trick is designed to break zone defenses and make blitzers hesitate. The receivers read the holes in the zone, run to them, and sit in them. If the quarterback can pass soon enough to keep them running, it's a bonus. If not, at least the play can gain about 5 yards. You'll see this a lot in the middle of the field in an attempt to keep linebackers from blitzing.
Nontraditional Targets: the Blocker/Receivers
One way to compensate for the lack of beef in the front five is to increase the number of defenders by reserving tight ends and running backs for pass-blocking. The additional blockers can provide double-teams, pick up blitzers, or act as a second level of defense for rushers who break through, but at a cost of the number of available pass targets. A very effective compromise is to have the tight ends and running backs 'chip' block – hit a defender with a shoulder on the way out to a pass pattern. This slows down the rush while still letting the player be a pass target.
But to really help the passing game, the use of tight ends and running backs as pass targets tends to increase in zone blocking offenses. Not only do these players lend blocking support, the defense is forced to account for them in pass patterns, thereby reducing the number of players available for the pass rush and buying the quarterback more time. This is the driving principle behind the substantial use of fullbacks and tight ends as passing targets in Kiffin's offense.
Fullbacks and halfbacks become particularly valuable in the passing game. Once established as a legitimate target, the defense is forced to count them among the targets and reserve pass defenders to cover their routes. If the running backs are then held as pure pass blockers (and not sent into receiving routes), the defense is suddenly wasting a defender on a vacated route and the offense gains a numerical advantage in the passing game. This buys time, which can then be used to allow deeper passing routes and stretch the field.
One feature you'll find in virtually every zone-blocking offense is the extensive use of the bootleg pass. The final method of avoiding pass rushers is to move the quarterback out of the way, and the play-action bootleg is supremely suited to this tactic.
Consider first: the play-action relies on an effective rushing attack to make the fake handoff believable. Not only must the running game be a viable threat, the initial play motion must look like a running play. This second element in a play fake is easier to create in a zone system than a man system. In a man-blocking scheme, the blockers lunge forward at the snap to meet the defenders as soon as possible; you can't do this on a play fake for fear of a lineman crossing the line of scrimmage. In a man system, the lineman motion must necessarily change on play fakes, which gives the defense one more opportunity to read the play.
In a zone system, however, the linemen take their first step sideways, and on outside zone rushes, the linemen often run sideways for a considerable distance before turning upfield. The sideways motion is the same on a bootleg play fake, so the defenders have a more difficult time sniffing out a play fake. For example, consider a play-action bootleg where the quarterback rolls to the offense's right. The fake will be a zone rush to the left where the running back and the entire offensive line head left, away from the quarterback. The defense has to respect the run and ensure that the cutback lanes are closed, which means the defensive line has to follow the rushing play – away from the quarterback.
Over the offseason, we've heard of the increased use of tight ends, running backs, and fullbacks as passing targets; the ability to bootleg has also received some attention (though to a lesser degree). Some mention has been made of the paucity of downfield passes in scrimmages; part of this is due to deficit (e.g. the injury to Denarius Moore), but part is because the passing game is built to complement the rushing game. The rushing game is the objective; the passing game is the counter – a counter that asks the quarterback to (a) make a quick decision from a limited number of options, and (b) to choose ball control over the risk of an interception. This is in keeping with the offensive philosophies that have been so effective for Shanahan's Denver Broncos and Carroll's USC Trojans.
In the following YouTubery, you can see all of the above principles at work. Watch the offensive line on the play-action bootlegs; the blocking motion appears identical to that of the NFL Films video linked at the beginning of the article (and here again for convenience). Between the two, you can see how the rushing and passing attacks are tuned to each other.
(Apologies for the quality, but this was the most illustrative video I found. There is no audio, either.)
So to Kiffin haters: yes, the lack of long passes during the preseason is all part of the plan.