This is an advanced defensive option where you can set a player to "shadow" a player on your opponent's team -- meaning your player will play whenever the target player on the other team plays. Your designated player will play even if it is not his scheduled shift in your to be on the ice with the other player. You player will focus less on scoring and more on preventing the target player on your opponent's team from having a good game. If your player is a good defender, he may keep the other guy off his game fairly well. However, if the other player is a good player, he could reverse the effect on your player -- especially if your player is a poor defender.