The Zone Coverage Rating is supposed to determine how well the defender plays/reacts within his zone. For example, I will call
Will Jack 3 and Ahmad Brooks [68 ZCV] will drop back into more of a middle hook zone instead of his assigned zone as through he were trying to help out Patrick Willis. Instead, he leaves his area uncovered and gives up a big play. He is a liability in zone coverage for me... but he has 90+ power moves and block shedding, so forget it. He will only blitz from now on!
Okay, so from now on Brooks is pass rushing/run support. Time to call
Strike OLB 2. Without user control, and on default setting, both Willis and Bowman think it is a good idea to just split wide and leave the middle of the field open. Why? Because the have very low Zone Coverage ratings. In fact, upon closer inspection, every linebacker not named Urlacher seems to have embarrassingly low zone coverage ratings. Linebackers have three assignments: Attack, play man, or play zone... and not too many are asked to play man. I find it ridiculous how low the ZCV ratings of linebackers are in Madden, so I took it upon myself to go in and increase everyone who had a rating of 65 or higher +10 points. That left Brian Urlacher with a 95 ZCV rating, and the highest in my rosters. The next best is like 92, and there are only like 6 players with 90 or higher. After doing this small change, it really improved game play in my opinion.
Additionally, I find that the ZCV also matters how much/hard a defender will chase in zone. For example, I will see a defender with a lower zone coverage rating follow a guy past his zone and a crosser will be left wide open. However, a player with a higher zone coverage will chase the first man to a point, release him and cover the crosser. They also take away angles better. For example, against a play like Colts Divide [where the TE runs a 10 yard out and the Z receiver runs a 5 an in] the SAM in a hook will back peddle and put himself in good position to bat down any pass to the TE outside, while the MIKE linebacker will position himself in such a way to prevent an inside throw. Then, as the Z recevier comes under, the SAM will let the Safety and CB deal with the TE as he clamps down on the crossing WR. He will close it off and let him pass to the MIKE/WILL and will return to his zone. At least, he will do that if his zone coverage rating is high enough. I also think that the reaction time slider MIGHT have something to do with that, but I am not sure. I am convinced that slider really only changes man coverage.
I have gotten many of the ratings in Madden 11 and 12 to "work" the best I can through slider adjustments and endless testing in practice mode and in game. There are a lot of factors that go into my settings and Madden 12. There is DPP with it's consistency, confidence, and player traits. There are slider settings combined with individual ratings. There are the plays you call and your execution... but I also mess with the speed threshold and fatigue settings as well. I play with like 90% fatigue/5% injury and custom substitution settings [No one comes off the field except for HBs and defensive linemen unless they are hurt. Linebacekrs sub out at like 3% and back in at 5% so they only miss a few plays at most.] That is a whole other topic you can find in my blog [link in my signature]