I thought this was the most appropriate since we are playing ND this weekend. It gives you an idea for the keys and rotation. There is considerable discussion for blitzes as well with a jump to the original story. [Emphasis mine]
Mod Edit: Click on the link if you want to read the full description of the plays as they are more interesting. Also, note: don't copy and paste full articles. Everything that's not a quote from here down is in place by the moderator, emphasis still his.
"This is a stop the run first type of defense. We want to outnumber the offense to either side of the ball. [...] The open side of the alignment is the flex side and the tight end side is the strong or solid side. [...]
The defense is the under/flex package used to outnumber the defense on each side of the ball by loading one side with linebackers while the other side gets safety support.
The Mike Linebacker plays the first back out of the back field to the strong side. The Free Safety plays the first back out of the back field to the weak side. [...] If we get a full flow by the backs to the strong side the Mike linebacker takes the first back and the Will linebacker takes the second back out of the backfield. [...]
Obviously the middle/mike is the first to the play, so he'll most likely be met with the blocking back who comes out first. This seems pretty obvious.
If it is a full flow weak side play the Free Safety has the first back and the Will linebacker has the second man to the weak side. [...]
In this case, the FS has the first blocker. Less plays are run to the weak side, which is why the defense is willing to risk the FS being the first attacker rather than a LB who can fight of a block easier and still make a tackle.
If the offense comes out in a different look such as a Twins look to one side, the basic core of the defense stays the same.
So it's easy to stay in base. Huzzah! Thus concludes this moderators attempt to show you how to quote articles.