biz and cbg are both mostly right, but there seems to be more to it. This is from the dev chat about the changes:
Each player will be evaluated by a position-specific formula that weights each rating relative to how important it is for that position. Then the player being redshirted will be ranked within the entire team roster, and his response will factor in that placement. Depth chart is not used for this, just listed position. Minutes in exhibition have no effect.
so the formula to rate players is position specific, (it rates every player on the team from the sounds of it) and then it compares that rating to the other players on the team (all of them apparently) and the kid you want to redshirt's decision is in part influenced by his rank in that...so its not just, ok he's the only listed PF so he won't accept it...I think...