I think one usage might be to create some forward momentum.

来源: Justme 于 2013-01-29 23:15:27 [] [旧帖] [给我悄悄话] 阅读数 : (170 bytes)

Murray steps forward for both 1st and 2nd serves. When the timing is right, it helps to take the ball early and reflect the power of the serving ball back to the server.

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