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Different coaches might offer differing answers to this question. My coach take on this question is that he wants me to straighten the arm slightly to straight on the end of the backswing in all forehand topspins. It depends on how much time you have. If you have a slower ball you have more time to straighten the arm more and go more behind you too. He stresses the need to be relaxed in the arm and start the forward hip rotation slightly before the arm swing so the arm straightens a bit naturally without thinking about it. So you do not try to straighten the arm at all, it just happens from the forward hip rotation and slight delay in arm motion. This can be hard to see in pro strokes as it is subtle. The relaxed form allows the wrist to move too at the end of the backswing too.