Zone blocking is garbage. There, I said it.
Bob Davie's explanation of zone blocking largely falls flat because for most of it, he's talking about what's called combo blocking. At its heart, this involves base or down blocking. But in most teams that run full-fledged zone blocking, it's a series of reaches.
Let me explain thusly. To oppose a regular five-man offensive line (no tight end), the two defensive ends will set up on the outside shoulder of the offensive tackles. In order for the play to successfully get outside, the tackle must come off quickly, take a flat step, and try to fight the DE to get his helmet across the DE's helmet. This is a reach block.
Here's the problem. The first commandment of DL play is this. DO NOT GET REACH BLOCKED. What's the second commandment? DO NOT GET REACH BLOCKED. And the third is this: DO NOT GET REACH BLOCKED.
We can begin on the chalkboard. There's a story about when Sammy Baugh was with the Redskins at the beginning of his career, and they were preparing for a game against the Eagles. The coach (whose name escapes me) drew up a basic play and said "This back will block the DT." Baugh raised his hand and said, "Coach, there's a problem. On Sunday that won't be an X on a chalkboard. That's going to be (HOF lineman) Bucko Kilroy." The coach scratched his head, then made a series of adjustments.
But in zone blocking, there is no real thought to the fact that defensive linemen and linebackers are simply faster than offensive linemen. If the scheme were truly that effective and revolutionary, there would be a huge gap in yards per carry that couldn't be explained by a simple talent disparity.
The reason why outside zone blocking fails is because, as more teams go to a read-and-react defense, all the DLs are fighting that reach all the way down the line. So when the ballcarrier goes to find a seam or a spot to cut back, he'll have all his own linemen, all the DLs, and at least one linebacker wedged into a very tight area.