I have recently had a whole new world open for me via using under and over defenses in the 4-3. I use the colts and have always fretted about the lack of a 46 defense in their playbook, but I am sim so I have kept trying to stop the run with a plain 'ol 4-3.
I had always stayed away from the over/under plays because I felt it "tipped your hand" as to what you were planning on doing, but I have found that the over and under are great for stopping unbalanced formations (such as the I) that people use for power running. There are also some nice zone plays in there to get some picks with as well.
Like most things I post, I am sure someone will say "Well, Duh!", but I really don't think this is common knowledge as I am sure many people had the same fears as I did about showing their hand or committing too much to one side. Spread the knowledge, save the world