Ya i would be curious to hear from JDB but i always thought it was more of a defensive mind set than an actual change in where they lined up. It's always better to hear from the actual game designer so we both don't give out bad info :)
Key run: slight boost to your defensive line in run defense, LB's and DB's play closer to the line and are quicker to get to the hole. Slight penalty to your defensive line in pass defense and to your DB's in the pass coverage attributes. Key pass: exactly the opposite of key run. Neutral: no penalties or boots.
