Key Elements of Run Support

Coaches:

Would you share your ideas on the key elements of run support: Force, Fill, Secondary Support and Pursuit?

1. I'd like to hear HOW and WHEN you apply these elements.
2. What basic principles do you use in teaching run support to your defense?
3. What keys do you teach your players to use?
4. Do you give a primary force call and a secondary support responsibility on each defensive call?
5. Under what conditions do you allow your safeties to audible to another call?

Thanks a ton!