JR Sweezy got what!?
I'm going to hop onto the cut-block conversation. I only read a few lines of it, but I want to add a bit to the cut-block discussion. It is pretty rare that even if a D-lineman doesn't go down, that he ends up making the play. It happens on cutbacks, but you usually cut on plays that are designed to go off the tackle's butt or further, so you're pretty far away from the hole the play is going to. You would never, for example, cut on a play up the gut. You've got to block that straight up.
The biggest reason you cut has nothing to do with getting a guy on the ground. You cut him to get in his head. To slow him down. You cut him and cut him and cut him every chance you get. Imagine you're a pissed off bull of a DT. You come flying off the ball, you think you're about to make a play in the backfield, and a tackle you didn't even see takes your legs out and you end up on your face. So you get more pissed. You come off harder the next play. And it happens again. Face full of dirt. Now you're really pissed. You might to start looking for it. You might anticipate it and start planning to put your hands down to push him out of your way. You might come off the ball a little slower.
Now...they snap the ball again. You're looking for the tackle to go at your shins. You're anticipating it, you've got your hands ready for it. Instead, you get a guard's helmet to the chin, and now you're on your heels. They run the ball right up your butthole. What do you do now? Do you come off the ball harder the next down? Do you lay back?
I personally wasn't the most physically gifted lineman in the world, and I was a good 30-50 lbs smaller than the DT's we played against, but man I could win that head game, and by far the biggest reason for it was I could cut-block like a SOB. I can't tell ya though how many broken ribs and bruised backs I got from big ol' dudes kneeing the crap out of me after they fell on top of me. I'd just laugh. Ohh, good times.
I do also want to clarify that the difference between man blocking and zone blocking isn't nearly as different as a lot of people understand it to be. On the majority of plays, the same linemen would end up blocking the same guys in the same directions regardless of which scheme you were using. It is more of a form of adjusting to blitzes and changing fronts than it is actual play design.