In your defensive game planning you can set probabilities for run/pass key.
If you are in run key, your DB's line up closer to the LOS, your rushers get off their blocks faster, and in general your defense has a bonus in terms of getting to the RB's hole quicker if the play is a run play. Similarly, the DB's play deeper and have a coverage bonus, and the DL has a pass rush bonus if you called a pass key against a pass play. If you call a pass play against a run play or vice versa, the bonuses go to the offensive advantage.