A slow pass rusher will allow the OL time to square up and block the entire pass rusher. A good pass rusher will be quick enough off the ball that the OL will only have time to get half of himself in front to block. By doing so, the pass rusher will be able to dip his own shoulder, rip under the arm of the OL, and press the pocket where the QB is, thus taking a shorter path to the QB. By getting to half a man, the pass rusher forces the OL to turn his shoulders, an action like opening a door, allowing the pass rusher to get to the QB quicker. In addition, an OL is not nearly as strong when only blocking with one of his arms, so the pass rusher can play with better leverage and dictate the play if he forces the OL into a weaker position.