Editor's note: As architect of Texas A&M's Wrecking Crew defense (1989-93), Notre Dame defensive coordinator (1994-96) and head coach of the Irish (1997-2001), Bob Davie has been recognized as a top X's and O's coach. This season, Coach Davie will analyze offensive and defensive schemes as part of his season-long course on football for ESPN.com. Each week, he will break out the chalkboard and break down the X's and O's in college football.

It is a necessity that every good offensive football team has a diversified screen package to combat today's attacking and blitzing defensive philosophies. Screens are an important element in successful offensive football for several seasons: (1) Screens take advantage of athletic wide receivers and running backs and make defenders tackle in the open field matching the wide receivers and running backs on linebackers and safeties; (2) Screens slow the pass rush and make defensive linemen eye for screens; (3) Screens attack zone blitz teams that give up an underneath zone in pass coverage; (4) Screens give the offense a chance for big plays against man-to-man blitz schemes.

The reason screens are so successful in college football is that offenses take advantage of a very important NCAA rule. In college football, offensive players are allowed to block downfield while the ball is in the air if the ball is caught behind the line of scrimmage. This is a major advantage to the offense because offensive players can actually pick defenders while the ball is in the air. Whether the defense is in zone or man-to-man coverage, this puts them at a tremendous disadvantage. (In the NFL, you are not allowed to block downfield while the ball is in the air regardless of where it is caught.)

Each of today's offenses feature several types of screens. We will explain the following screens: (1) the bubble screen, (2) jailbreak screen, (3) the traditional slow screen, and (4) the crack screen.

Bubble screen vs. 3-deep or soft man-to-man
The bubble screen is a wide receiver screen where the receiver actually bubbles away from the line of scrimmage and the quarterback. The most common form involves the other wideout picking the defensive back and giving the receiver a chance to run after the catch. The wide receiver bubbles back to allow this to time out. This is a great route against a 3-deep zone or soft man-to-man if the outside defender is giving a cushion.

It's a simple scheme, but the throw is not as easy as it looks. The quarterback must throw it accurately so the wide receiver can catch the football in full stride on his way toward the line of scrimmage. You actually may turn the outside defender loose (not blocking him) and just see if he can tackle the receiver in the open field.

Bubble screen vs. zone blitz
The bubble screen is also used to combat today's zone blitz schemes. If the defense blitzes and goes from a 2-deep hide to a 3-deep, it is a great play. Some offenses will actually abort a running play and throw a bubble screen if the linebacker blitzes. The only players who make this judgment are the wide receivers and quarterback. The offensive linemen and running backs actually go ahead and execute a running play. What the offense has done in the huddle is call two plays. In our system, we called this a 23 alert bubble (23 indicates a running play). The call allows the quarterback to audible to the bubble screen at the line of scrimmage.

The jailbreak screen
The jailbreak screen is a wide receiver screen that involves the wide receiver coming back to the quarterback at the snap of the ball. The reason it's called a jailbreak is because the offensive line releases automatically downfield to block. The offense uses a tight end or wide receiver to go away from the line of scrimmage to pick the outside receiver's man. The linemen punch the defensive line to stop their initial charge then release downfield to form a wall. The offensive tackle stays in and chops the defensive end to get his hands down so that the ball can be thrown over the top of him.

The conventional or slow screen
The least commonly used screen in today's football is the slow screen. This play requires a lot of timing and execution and involves a lot of deception. It is effective against zone defending teams and defensive linemen that get up the field causing separation. It is generally not effective against man-to-man as a linebacker immediately zeroes in on the running back.

The quarterback, instead of taking his traditional 5-step drop, actually drops deeper to allow the defensive linemen to rush up the field farther. This also allows the linebackers to drop deeper into coverage which creates separation. The offensive tackle to the side of the screen sets as if to pass block and then chops the defensive end -- once again so the ball can be thrown over him. The guards and centers hold for a one-thousand-one, one-thousand-two count and then release to form a wall -- usually to a landmark on the field. The halfback stays in, fakes pass protection and then slips out to be the receiver. As you can see, it takes much timing and execution, but if you catch the defensive in zone coverage, it can be an excellent play.

The crack screen
The crack screen is a tremendous play against man-to-man coverage when you know which defender is assigned to the running back. You simply crack a wide receiver from the outside in on the defender responsible for the running back. Once again, you are allowed to block back towards the ball as long as the ball is thrown behind the line of scrimmage and you don't block below the waist. In the example below, the wide receiver cracks back on the mike linebacker who is assigned to the back. As simple as this play looks, it is difficult for the man covering the wide receiver to switch off and take the mike linebacker's man.

Crack screen vs. man-to-man
In this particular play, the wide receiver cracks down on the linebacker who he knows is responsible for the running back in man-to-man. Again, you see the offensive tackle set and chop the defensive end and the center of guards release out in front of the pass receiver.

Summary
As you can see, there are a variety of screens to choose from. The success of the offense depends on calling the right screen against the defensive matchup. With the wide receiver screens, it is a simple execution play and a lot depends on the ability of the wide receiver to run after the catch. The big positive in screens is that they are basically simple to execute and provide you with the chance for a big play against an attacking defense.

Q & A with Bob Davie
First of all, thanks for all of the terrific responses and knowledgeable questions this week. I wish we had time to answer them all. Please keep sending in the questions and we'll tackle as many issues as we can this season. Here are a few of your questions regarding the zone blitz:

Coach,

The zone blitz packages seem to require a premium effort from strongside linebackers and the safety positions when it comes to reading a play, utilizing the speed to get there and then, sure tackling ability to boot.

Is this assessment correct? And if so, could you provide some examples of pro or college players who excel in these particular schemes?

Tom Puro
San Diego, Calif.

Bob Davie: I think it's obvious that good coaches find a way to do things with their difference making players and take advantage of their abilities. A lot depends on what type of players you have and putting them in a position to be successful. Good coaches design their schemes to take advantage of their best pass rushers and blitzers. Your assessment is correct. In the figures I used in last week's discussion of the zone blitz, I did highlight the strongside outside linebacker and the strong safety.

Watch the games every Saturday in college football and you will see that most defensive coaches will put their best defensive players in a position to wreak havoc on the opposing offense.

When I was at Texas A&M, we really took advantage of former quarterbacks and running backs, converting them to linebackers. Players like William Thomas, Quentin Coryatt, Aaron Wallace and John Roper gave us speed at outside linebacker and allowed us to blitz effectively.

Hey coach,

Unless it is Julius Peppers, isn't it asking a lot of the defensive end to cover a 1/4 to a 1/3 of the under coverage? Isn't that where an offense should go to exploit the weakness in a zone blitz?

What's to stop the X from running off the corner with a go and then sending H on a flat route or a wheel up the sideline? Most defensive ends shouldn't be able to run with a running back.

I can see the zone blitz being effective as a changeup, but if a defense relies on it too heavily, I would think it creates a pretty big mismatch for a smart offensive coordinator.

How often did you employ the zone blitz on passing downs?

Tim
Tumwater, Wash.

Bob Davie: I think you bring up a tremendous point. This week's column on screens shows a great way to combat the zone blitz. It's also great to throw in the zone where a defensive lineman is assigned in coverage. One positive of the zone blitz, though, is that the ball has to come out so fast. Normally, the offense doesn't get a pre-snap read that the zone blitz is coming. The defense depends on that surprise element.

Offenses have evolved -- everything does run in cycles -- and are starting to do a better job against the zone blitz. As a result, you're starting to see less and less of it.

Great column. This is just about the best thing espn.com has ever done, in my opinion.

I'm wondering, with regard to the zone blitzes, under what situations a defense would opt to use a given blitz. They all kind of look similar to me, but would there be certain game situations that a defense would blitz a safety as opposed to a backer?

Also, when would a defense use a 3-deep zone as opposed to a 2-deep?

Bruce Hansen
Orem, Utah

Bob Davie: Actually what happens is that the zone defense scheme has evolved to where defenses will check from one zone blitz to another depending on the offensive formation and tendencies. There are definitely more zone blitzes that are more advantageous for certain formations and against runs or passes. The defense tries to check into those. You are right, with every zone blitz you call, there is a tradeoff. What you try to do is check into the zone blitz that gives you the best opportunity against the formation the offense is in or what play you think they will run.

Coach Davie,

Just curious, in your version of "SLB zone blitz", you have a tackle dropping back to cover in the middle for a linebacker. Do you expect him to be effective in this or do you just get a big body in there that can disrupt the flow in his zone? And if it is a pass play, can he really get back there fast enough to make a difference? Thanks

Nashvol
Nashville, Tenn.

Bob Davie: The biggest reason that you would drop the tackle into coverage would be in case it is some kind of screen pass and you just don't want to give up a big chunk of the field. We actually did that zone blitz with a 3-underneath, 3-deep concept many times -- particularly on a running down. We would not ask the nose guard to have any pass responsibility at all. If you are calling that on a passing down, instead of just voiding the middle of the field, you may ask the nose guard to drop out late. If it's a running down, you probably void it completely.

Send in your Football 101 questions. Bob Davie will answer a few of them next week.