cover 2 keys

I learned that corners read #1 to #2. They collision #1 and force inside, then sink with #1 to 12 yds if he goes vertical and get your eyes on #2. If #1 run inside, yell "IN, IN, IN" and look for #2 running the out. If they flood the zone (EX: curl / flat) split the difference but be sure to take away the curl up to 15 yards up. You can always come up and make the play. I tell my guys, when in doubt, be able to make a play on the widest and deepest guy in the flat.
The curl player collisions #2 and reads #2 to #1. If #2 runs an out, yell "OUT, OUT, OUT" and look to collision #1 running inside.
Safeties (deep half) must be deep enough and wide enough to keep everything in front of them and make all plays down hill.

As a general rule in any zone, it's a good idea to get depth. If you're an under coverage player and there is nobody in your zone, drop to 12 so you can see them coming in and collision.
As a deep half player, you must keep every one in front.
Not sure if that's what you were looking for.