A 3-4 defense is only successful when you pressure a QB. Watch the Pittsburgh, Baltimore, Houston, Green Bay, New England, NYJ. When they don't get pressure, their back end is exposed. When they get pressure, they are a carnivorous defense that just sits on the short stuff because they know the QB doesn't have time to set up. But when the pressure doesn't get home, and the QB has time to set up, they get torched.