Rumors have been running rampant that the San Diego Chargers intend to switch from a power-blocking scheme to a zone-blocking scheme for their offensive line since the hiring of Joe D'Alessandris. Why? Well, D'Alessandris ran a zone-blocking scheme with the Buffalo Bills after running it for years at Georgia Tech. Considering that neither of D'Alessandris' bosses have any experience with coaching the offensive line, I can't imagine that he was brought to San Diego to learn how to run the power-blocking system that the team used under Hal Hunter and Norv Turner.
I have a very basic understanding of the differences in Power Blocking Schemes (PBS) and Zone Blocking Schemes (ZBS), but it does seem comparable to the differences between a 4-3 defense and a 3-4 defense. The 3-4 defense relies heavily on coaching, working to confuse opposing offenses rather than try and beat them with skill. This takes some of the burden off of the General Manager to put above-average athletes at every position. The 4-3 defense requires good coaching and smart players, but it can not be effectively run without great talent at just about every position.
PBS is just what you would think. Five offensive linemen walk up to the line of scrimmage with a plan. They each have a guy that they have to block. On a running play, they each have a guy they need to push back. Teams with outstanding power along the offensive line (such as the San Francisco 49ers) can run this scheme without issues and reap huge benefits from it.
ZBS is a little more complicated and does more to confuse defenses. There are more creative double-teams on defensive linemen to open up holes. It can sometimes keep defensive players from penetrating simply because they have to worry about getting caught in a trap set by the offensive line, which is pretty much the basis of how ZBS works to stop opposing pass-rushers.
Here's a fantastic video from Fox Sports that breaks it down as simply as possible in about a minute:
Just as Billick says in the video, PBS requires "good players" and ZBS requires "good coaches". The beauty of ZBS is that you can turn an offensive line from very bad to quite good in a hurry, because the players needed to make the ZBS work aren't necessarily sought after free agents or high draft picks.
If the Chargers are switching to ZBS, what does that do to their offensive line? Most of the guys signed to contracts with the team are PBS linemen, which means they're big and slow-footed rather than small and athletic. That could either make it a rough transition in terms of performance, or D'Alessandris will need to run a hybrid scheme of sorts until he has enough ZBS guys to run things the way he wants.
Here's another YouTube video of the Texans offensive line, at half-speed, that will show you what a basic run looks like with ZBS:
You can see the entire offensive line moving to the right, making reads along the way as they move from the first-level defenders to the second-level defenders the same time that Arian Foster is making that transition. The ZBS takes great coaching, smart players, great athleticism along the offensive line and a ton of practice to get the timing right. It forces defenders to make a decision, left or right, and then takes advantage by going the opposite direction. When the ZBS works, you can take undrafted free agents like Arian Foster (or Ryan Grant or Fred Jackson, etc.) and turn them into All-Pro players.