I have read enough of these post and found them helpful and enjoyable. I thought I would try to contribute to the cause. Here is my first post.
You can find the TCU 1999 playbook here. There is not a whole lot of detail on the coverages but you can get a good idea for what they are trying to do. I have never visited with the coaches at TCU, only talked with others who run similar 4-2-5 defenses, so the terminology and alignments will not be exact, but within the general idea. I am going to discuss how to utilize a split safety coverage system versus spread offenses (2x2 and 3x1). These will be called out of 4-2-5 personnel. These will be different for a 4-3, because the COS is difficult.
In defense it helps to begin with the ordering of a coverage call. In this scheme the front call is irrelevant to the coverage the secondary is given. The call I will be discussing is 2-BLUE SOLO & 2-BLUE SPECIAL
The "2" Refers to the read side coverage. The read side is the side of the passing strength, and is the side that the SS and FS go to. The read side safeties will relay the coverage to the read corner and linebacker. The "BLUE" call is the away side coverage. This is the side away from the passing strength. The WS will let the away corner and linebacker know what the coverage is. The Diagram below shows it.
Cover 2 in this scheme is not the typical squat/halves (cloud) coverage that is quite common. Cover 2 is a robber scheme. There is a post about Virgina Tech's Robber coverage here. TCU's and Virgina Tech's are very similar.
To summarize robber. The FS will align o/s shade of the OT and 10 yards deep reading the linemen. On the snap of the ball the FS will remain flat footed until he get his read, on a run read he pursues the ball inside out. If he reads pass, his eyes flash to the #2 WR to the read side.
If #2 releases vertical past 8 yards the FS will yell "Push, "Push" and takes the WR man to man. If #2 releases out or in within 8 yards the FS will rob under #1 looking curl, post, dig.
The SS is responsible for the out cuts by #2 and if he gets a "push" call from the FS he will move to get under #1. The read corner aligns at 7-8 yards off LOS and bails playing a 1/2's technique over # 1.
On the away side the WS and away corner are playing cover "BLUE". This is a type of combo man between the two of them. Both are reading the # 2 WR.
If the # 2 WR pushes vertical past 8 yards the WS takes him man to man and the corner locks up on #1.
If #2 releases outside within 8 yards the WS will make a "wheel" call to the corner. On a wheel call the corner Comes off and takes #2 man to man, and the WS locks up on #1 with over the top leverage.
If #2 releases inside the WS will yell "IN" "IN" to the linebacker and then double #1 with the corner.
The read and away sides respond differently to outside cuts by the #2 WR's. The picture below shows how each side will respond to the curl/flat combination.
The final word of the call is SOLO. This is the trips coverage. This alerts the secondary that on any 3x1 formation the away side will check into SOLO.
SOLO is a trips check aimed at defending the trips side at the expense of putting the Corner on the single receiver side on an island man to man. The Read side continues to play Cover 2 on #1 and #2. The handeling of the #3 WR is done by the read side backer and WS. The WS has force/ pitch to his side. On pass, the WS sprints toward the middle of the field hunting for the # 3 WR. He has him Man to man. The read side linebacker's job is to wall #3, re-routing him and preventing a quick throw.
Versus a split play the coverage looks like this.
On the read side the FS makes a "Push" call and takes #2. The SS gets under #1, and the WS sprints across the field and takes #3. Below is another example.
This is an example where running 2-Solo is perfect. The play is designed to attack the trips side. The FS sees #2 release o/s and then robs the curl of #1. The SS drops to the flat and picks up #3, and the corner is deep to handle the wheel route by #2. If the QB is reading the corner and SS he will throw to the curl not realizing the FS is getting underneath it.
CHANGE OF STRENGTH (COS)
COS deals with motions that change the read side. This is one of the most troublesome things to deal with in this system. However, the presence of a third safety makes adjustments easier. Thats why this system is more difficult to run out of a 4-3 or 3-4. If the read side changes the FS moves to the other side, and the SS and WS swap responsibilities.
The read side flips to the right. The FS and WS play cover 2 and the SS checks his side into SOLO. After the motion the secondary is aligned like this.
The beauty of this is the adjustments were made by the secondary. The linebackers don't really have to move, only change responsibilities.
ANOTHER TRIPS COVERAGE "SPECIAL"
SOLO is designed to Load up on the trips side. You can run multiple coverages to the trips side with SOLO on the away side. You can put your read side into squat/halves, man, and BLUE. SOLO allows for flexibility in coverages to the trips side.
However, you might not always want to lock up on the single receiver side. You need a coverage call that allows for flexibility to that side too. That is where "Special" comes in. If the coverage is 2-BLUE- SPECIAL, your secondary checks into special with any 3x1 set.
Special puts the read side corner man to man on # 1. The SS and FS play #2 and #3 with BLUE coverage (Treating them like #1 and #2). The SS acts like the corner does in Blue and the FS acts like the WS. The other players disregard the #1 WR the corner has man to man. (This is an X-out concept. You could use a similar adjustment VS TE trips (trey) You can lock the SS on the TE and have the FS and corner play blue on #1 and #2.) This locks down that side and allows you to play games with your WS and away corner on the single receiver side. You have many options here. You can run different brackets, play squat/halves, Spy the QB with WS, or even send him on a blitz off the edge.
You can add a 4th tag to the call to let the WS and Corner know what to do vs trips, or have the WS look to the play caller for a call when he gets trips. It helps to have a base call for the away side in special. A simple bracket call works best for starters.
Special is also the call of choice versus an empty (3x2) set. Versus empty the FS checks Special to the trips side and the WS gets the away side into blue. This is a good coverage it provides great run support for draws and get all the WR's covered.
With 2, blue, solo, and special you can present a number of looks to the offense,have a plan to deal with the trips side and open side in 3x1 sets, and even cover empty (3x2) sets. The ability to play with 5 players in the secondary makes adjustments and communication simple.
I have written another post that goes into alignment details of split-field coverage versus 2x2, 3x1, empty backfields.