I think there is an explanation at the official wiki. It says that when a player's SA is active, he performs as a player of one level higher. About the shadow striker, I'm not sure but I think it means that that player is good when he shoots from distance, which means his attack abilities will be increased by 5 points when he shoots from distance. This is one of the best SA as shooting from distance is a very common thing for an AM. A defender with a Defensive Wall SA will have his defense capabilities increased by 5 when the opponent is shooting a free-kick, especially when crossing. Anyway, this ability is more necessary when the opponent has a free-kick specialist.