In the past four installments we have covered several different types of coverages, now we are going to delve into the world of combination coverages. Combo coverages are exactly what they sound like, the defense plays a combination of coverages. Any two high coverage can easily be split in half and combined with another one. This is almost never done with cover three or one. By taking existing plays the defense knows and mixing and matching them a defensive coordinator can provide a lot of looks with relatively little teaching. TCU, for example, has been doing this stuff for a decade plus now. We are going to delve into some of the more common split field coverages.
The first and most common coverage is known as either cover 6, quarter quarter half, or 42 (cover four, cover two). We have already established how similar cover four and two are, this coverage combines the two. Nothing changes for anyone but two safeties and two corners. The quarters side of the defense is usually set to the field. Setting to the field ensures that the weakside linebacker has less space to cover, allowing him to play in the box or at least closer to the box.
In the middle of the field the quarters will be played to the passing strength. In this situation the weak linebacker may well have to leave the box to cover his zone. To also account for a gap the linebacker will have to give a call to the defensive end (my team used "crash" for example) that will inform the end the take the B gap while the displaced linebacker takes the C. The weakside linebacker has to, in the words of my position coach "yell crash until the defensive end smacks his ass (the signal for "got it") or go up and smack that ass hard enough the (fill in as much cursing as you want here) end keeps his ears open next time." That man is 5'4", has a mustache Ditka would respect, worked construction and is everything a linebacker coach should be. Disadvantages of setting to the field basically boil down to teams overloading the boundary side with receivers, in which case an adjustment is needed. Switching the quarters side to the passing strength can mitigate this, but also leaves a lot of open field for the field side wide receiver to work with.
Teams can run cover four or two to one side with man to the other side. This will mitigate situations discussed above, where one receiver can have a huge chunk of field to work with, but also has potential to force your linebackers into man coverage on wide receivers. Defenses can choose to keep the linebacker in the box (as a blitzer or QB spy, both of which teams often don't expect versus cover two/four), at the expense of forcing the safety and corner into a cover zero situation on one side of the field. This is also a vulnerable coverage versus crossing routes, with their being no one playing zone to one side of the field, if a crosser can get over there there will be problems. Running backs releasing into the flats to the man side can provide issues as well.
For more on split field coverage defenses look into Brophy's no huddle defense or this copy of theTCU defensive coverages from 1999. If you want to read further into quarters, the 4-2-5, or 2 read look at Coachhoovers excellent 4-2-5 resources