Defending the Pass With the 3-4 Defense, by Coach Jeff Reinebold, discusses how the 3-4 defense provides a team with some unique opportunities to put pressure on the quarterback-physical pressure by hitting him and mental pressure by giving him multiple looks and multiple disguises.
The net effect of the multiplicity of factors that the defense is giving the quarterback is to make it very difficult for him to know exactly what the defense is going to do after the snap.
Coach Reinebold details the key factors involved with employing the 3-4 defense against the pass, including reviewing the advantages of the 3-4 for attacking the pass game.