Over/under defenses simply refer to the shift of the DL to the strong or weak side, respectively, and the rotation of the linebackers in the opposite direction. I was bored the other day (waiting for a flight out to an interview) so I started scribbling various notes on the advantages and disadvantages of these shifts.
An over D puts the ends directly over the weak tackle and the tight end. One DT is over center, and the other is between the strong guard and tackle. What this does, if the TE goes out, is it forces the offense's double-team to occur on the weak side of the line. Assuming a strong right formation, we have the RT moving over to block the LE and the RG isolated on the LT. The C has to take the RT, the LT has to take the RE, and the LG is free for a double-team. Contrast this to a standard defense, where the center is free to double either DT, and you see the benefits if there's a mismatch on one side of the line. Under D will create the same one-on-one blocking on the other side of the line. Additionally, an over shift will jam the TE at the LOS with your end, and an under shift may have the SLB jam him. Expect the tackle who lines up over center to rush away from the double-team on a passing down, and towards it on a running down. More to the point, if you're controlling that tackle, you probably want to do that yourself. On a run, you want to draw the double-team to allow the LB's to plug the holes and make the play. On a pass, obviously, you want to create the one-on-one for your three best linemen and hammer the QB.
I also had some thoughts on how to pick on over/under defenses. First of all, using max protect blocking (TE stays in) will go a long way towards defeating an over. Second, notice that the LB's are already rotated away from the line shift. They're also pulled in towards the center somewhat more than they would be in a base 4-3 set. This has a couple of effects:
1) The outside running game away from the line shift will open up somewhat. The end is easier to seal off on a quick pitch, which leaves one lead block on the LB between you and a 5 yard pickup.
2) You're not going to get LB help on the WR that the line is shifted towards.
3) [Extension of (2)] If the D is playing a cover 3 zone, the DB's will most likely rotate opposite the linebacker rotation. I.e. if we're in an over defense, the LB's are rotated to the weak side, so the DB's rotation in cover 3 will most likely be to the strong side. If they rotate the way you're not expecting, the rotation will leave huge holes as people try to cover extra field to reach their zones. Either way, you should be able to exploit the zone effectively.
4) Against man, you'll want to throw the ball quickly. The help is father away than in a base--get the ball there and trust your WR to win the one-on-one.
I also came up with a custom play that should illustrate some of these principles. It may be similar to things in the M2K playbooks (probably is, but I did diagram it out of my head in an airport). Here goes:
Regular I, normal set
QB: 3/5 step drop (depends on who your QB is)
HB: Flat L
FB: Pass Block (your choice)
SE: Corner 5
FL: Cross L
TE: Drag R
OL: Pass Block (your choice)
Here's how it works:
Against an under D, the SE should be open on his quick corner, and the TE should be open in the flat fairly quickly on the other side. Against an over D, the FL on his cross will come open against zone, and the SE will again be open against man. If the D is normal, or nobody is getting open, dump it to the HB in the flat, or throw it away. --Bill