Use the same ready-position as you would with an overhand float serve, with your feet shoulder width apart and slightly staggered.  Your weight should be resting on your back foot and your body leaning slightly forward. You will hold your shelf arm out perpendicular to your body to throw the ball. Bring your hitting arm back with your elbow pointing behind you, at about eye level. Throw the ball up into the air like you would for a float serve, but throw it at least 18 inches (45.7 cm) above the starting point.  Be sure to throw it straight up, not to the side, to allow for a balanced hit. Although you are throwing it a bit higher for the topspin serve than for the float serve, do not throw it too high. You will likely under/overestimate the timing to hit it, leading to an unbalanced hit. Use the same pose as you would for a float hit, with your elbow above your ear and behind your head. Instead of punching at the ball like a float serve, you will instead hit downwards with an open hand.  As you swing, you will turn so your throwing shoulder is facing away from the ball.  Snap your wrist downwards so that your fingers are angled towards the floor. Do this as you make contact with the ball to push it forward. Follow through all the way with you arm on this serve, so that your hand will come to a stop much lower than where the ball originally was. You will end the hit with your weight shifted to your front foot.

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Write a summary of the previous text in Chinese:
先站好姿势。 抛球。 胳膊回拉击球。 向前挥动胳膊击球。