Explain Focus?

In the defensive matchups what is the focus checkbox vs assigning a player to cover a spacific opponent. If you do one do you want to the other or only one or the other?

[MJAY2003]

I'd like to know the answer to this one also. I haven't noticed any difference when focusing on a player and not focusing, but I may be doing it wrong by focusing on him and also putting a defender on him.

[dunelly]

Zones shadow the focused player. i've tested it, if you want to, just call 2 man and you'll see the safeties focus on the star player.