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This emphasizes opening the hip up first, and snap back towards your rear end, on the forehand. I call it the snap back, wherein, as you move into the shot, with the hitting foot, the snap back occurs, which creates a whipping effect. Body moves into shot, while wrist snaps stick back towards your own ass. Fed does this, and goes from low to high in the stroke, and creates a ton of pace. He also disguises the shot, and acts as if he is always going inside out, and freezes opponents, so that they cannot react/read/anticipate the actual direction until it's too late.
Hip rotational speed in elite players can reach upwards of 400 degrees/per second in the forward swing. Hip rotation increases overall range of motion and speed of shoulder rotation capabilities. Top players always open the leading hip up very fast when they go for a finish or a jam shot.