Let's look at the anatomy of a blitz. When a defense blitzes, they are hoping for two main things. First, the offense becoming rattled which they hope will cause mistakes, and second, trying to rush one more defensive player than the offense can block which in turn they hope will allow the defense to get easy sacks.
With the abundance of offensive packages that utilize two tight ends, and at the same time moving to many variations of the spread by almost eliminating the fullback, the defenses in the NFL have adapted by replacing one of the linebackers with an extra defensive back, (usually a cornerback), which becomes their base package.
Recently we have seen what is called the "big-nickel" defense instead of the regular nickel. The difference being substituting a safety for the third linebacker instead of bringing in an extra cornerback.
The extra safety, (who is usually just a big corner), can better match-up with the new hybrid-tightends like Jimmy Graham and/or can still match up with many of the bigger receivers. This allows the defense to line up in the 4-2-5 and the 3-3-5 packages, (among others), and still be able to send many exotic blitzes out of these alignments. Keep in mind that one of the things this does is to keep "gap-integrity" while still sending more than the offense can block.
Remember, as far as gap-integrity goes, there are six gaps when there are five offensive linemen and seven gaps when the offense adds a "tight" end. The defense is asked to take their front six, (the 4-2, or the 3-3), in the case of the five offensive linemen, or the (4-3 or 3-4), when the offense has a "tight" end, and then with these defenders fill all the gaps. Typically two A-gaps, two B-gaps, and two C-gaps and then one D-gap if the tight end is lined up tight to yield the six or seven gaps to fill.
Notice in the case of the 4-2 or 3-3 we have six defensive personnel attacking five offensive linemen if all six rush the quarterback, (if the tight end is spread out or goes out for a pass). When the offensive coordinator sends the tight end out for a pass, then the running back is often asked to pick up the sixth rusher if the defense sends him.
The defense will try to trick the offense by faking a sixth rusher or will sit and wait for the offense to commit to the running back staying in to help in the blocking scheme for that play. This situation, (waiting for the running back to commit), allows the defense to use the "green-dog" blitz.
While the "red-dog" usually just means a defender such as a linebacker blitzing, the "green-dog" is basically where a defender, (also usually a linebacker), is assigned to cover a running back out of the backfield, but is given the "green-light",(or asked), to blitz if his man stays in to block. This is also why "Max-Protect" can be problematic, (it encourages green-dogs to go right where the quarterback is).
The best way to defeat a team that employs a lot of green-dog blitzes is to leave the running back in to block just long enough for the green-dog blitzer to commit and then send the running back out on a wheel route with the outside receiver blocking his defender away from the sideline which can produce a big play. And that is what often happens to blitzing teams, that is, big plays.
The really smart coaches have in their playbook what is call the "offensive blitz" play where instead of running out the wheel-route, the running back stops and throws to the quarterback who has "leaked-out" towards the other sideline and either takes off running or throws a deep pass that has had more time to develop. The coaches that do this play will often keep it for a playoff game.
" The New England Patriots and Pittsburgh Steelers have run variations of the play [offensive blitz] successfully in the National Football League playoffs." - Wikipedia (bracket info added by me for clarification.)
A quarterback is usually taught to read the free safety first and then the other defensive backs and then finally the front seven. He will then call out who the Mike linebacker is to insure the offensive line is in sync on the blocking assignments since the Mike is usually the key to those assignments.
Jim Haslett is very good at using the green-dog and he loves to use cover-one for much of his blitz packages as well as cover-zero. He uses a lot of blitzes on any given team, but against the Cowboys in week eight, he went blitz-happy and blitzed on about 60% of the defensive snaps.
The screenshot below shows one of the Redskins use of cover-zero and the green-dog.
Even though it does not start out as cover-zero, the free safety comes down in the box to cover the tight end and that is what leaves the deep middle open, and causes the cover-zero alignment. (Cover-zero means no deep safety).
By having the linebacker cover the tight end sometimes and the safety covering him other times, it helps keep the quarterback guessing what coverage he will see on the tight end.
If the running back had stayed in to block, then the green-dog safety, Phillip Thomas, (red circle), will come on the blitz.
In FIG-2 we see that the single safety can either drop into a deep zone or can come up. FIG-3 shows that when he comes up, then it becomes a cover-zero.
And finally FIG-4 shows that if the green-dog comes and he neglects the running back, there is an opportunity for a big play. Oh, and FIG-4 also is an example of cover-zero.
Here is an article from over at the mothership that also talks a little about the heavy blitzes.