The question is not if Luke's saber is behind vader's. After all Luke swings his saber from upper right down to the left. I think everybody agrees Luke's saber is behind vader's and closest to the emperor. The discussion was if that should be the case. If you think Luke wants to hit the emperor then his saber shouldn't be behind vader's, because vader could not have blocked it. If you believe (as I do) he wants to hit vader then there's nothing wrong.