That's where power and topspin come in.

这就是力量和上旋进入地方。

Since a curveball has topspin, that means the air behind it gets deflected upwards.

由于弧线具有上旋， 这意味着它后面空气向上偏转。

Man wouldn't know topspin if it hit him in the ball sack.

中下身才知道上旋害。

However, it has a high topspin, and it's awkward to return.

但是它上旋较高，回比较困难。

But topspin serves are more accurate and still difficult to return.

但上旋发更准确，而且仍然很难回。

That's why Serena also uses topspin to her advantage, usually when her first power serve goes out of bounds.

这就是为什么小威也利用上旋来发挥自己优势，通常是在她第一次强力发出界时。

That's because the topspin on the ball forces it to drop sharply right before it reaches the opponent.

这是因为上旋迫使它在到达对手之前急剧下降。

So, Serena could be about to deliver a topspin serve, a power serve, or even a slice serve.

因此，小威可能发出上旋发、强力发，甚至是切发。

For example, Serena's average topspin serve is about 145 kilometers per hour, 25 kilometers per hour slower than her power serve.

例如，小威平均上旋发速度约为每小时145公里，比她强力发慢每小时25公里。