There is some complex logic in the change player action depending on what is happening. Generally speaking it won't change you to a player who has no chance of catching the ball carrier for example.
Out of interest, how do you rate this game? Compared to R08 and RL3 for example. How do they play compared to them? And do you see any problems there that people might pick up on xbl and find easy ways to beat defences? Cheers for the time.