From Jeff Cooper,
Your Guide to Tennis.
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Shortly after contact with the ball, Roger's racquet is facing downward more than usual here, probably due, at least in large part, to his hitting this forehand with his weight on his back foot. Most players tend to pull across the body and turn the racquet more on the follow-through when hitting off the back foot, because when you hit off your back foot, you can't drive forward into the ball as much, so the follow-through naturally goes forward less. One way to compensate for being unable to drive into the ball is to hit heavier topspin. Judging from how much Roger's racquet has risen in comparison to the height of the ball, he hit this shot with heavy topspin. When you want to hit a high, deep topspin, you'll often deliberately lean on your back foot to create just enough tilt in your stringbed to give the ball a high trajectory while still brushing upward for the spin.