According to Tactical Tennis by Glen Hill, Nole stopped "scooping" as often on the follow through and became less vulnerable to the low slice Fed used to great effect in beating him at US Opens past. . I always thought that when Djoko opened his racket face on the backhand, he was simply driving the ball more. But, hey, that's above my pay grade <g>. My pics from two years at Indian Wells.
Excerpt: "Djokovic made improvements on the backhand side in two ways. First he started to close the racket over the ball both more and more often. Whereas in the past he was given to scooping the ball on his backhand side with greater frequency and seeming at random, he really cut down on the frequency with which he did so. The second adjustment is he started being far more intelligent about which balls he scooped on. As mentioned above the scooping action flattens the contact out and takes away topspin. The resulting shot will also skid through the court more. It is far more effective when done on higher balls, when less net clearance is needed for the ball to come down into the court. The margins are greater on that shot, and it can be hit almost down-hill as a driving flat ball, particularly down the line. Whether by intention or accidental habit, Djokovic now does just that. He moves to the ‘bad’ backhand typically only when either stretched and short on options, or on those higher balls he can afford to be genuinely aggressive on."