After 8 seconds the referee puts his arm in the air signifying that the visiting team's line change is done and that the home team now has 8 seconds to make their change. In cases where a substitute player is not allowed to take the ice prior to the faceoff, most often this is happening with a visiting player changing when the referee's arm is in the air and that team can no longer make a change. That player is sent back to the bench and the players on the ice must remain for the faceoff. Once the puck is dropped, that player will most likely change on the fly. Line changing at stoppages has alot to do with coaching strategy, as faceoffs are crucial game situations.
Coaches try to match lines.
For example a visiting coach may put his high-scoring forward out to win a crucial faceoff and the home team coach will see that and put his best defensive forward out to stop the opposing player from scoring. When this happens, the visiting team may try to sneak the high scoring forward back to the bench for another player, so he can skate the next shift without have the defensive forward shadowing him.