Without warning, Bardo smacked his fist into his open hand.

毫无预警地，巴多用拳头啪地猛击了他张开的手。

《柯林斯英汉双解大词典》

We assume it's because it's a fist, which is undeniably more badass than an open-hand slap (paper) or angry pointing (scissors).

我们假设这是因为当你出石头时，你的手是个拳头，不可否认拳头比伸开的手掌(步)或是愤怒的手指(剪刀)更有气势。

youdao

Mr Obama has been on a goodwill tour of the world, proffering the open hand rather than the fist.

奥巴马的国事访问跟多的是向世人表达了良好的意愿，是以握手而不是拳头的姿态。

youdao

He cannot open the fist to look at the thought, for fear that it will fly away, but he knows that it is very important and that he must hang on to it, no matter the cost.

NEWYORKER: The TV

Open hand or iron fist?

CENTERFORSECURITYPOLICY: What Undid the Middle East ��Peace Process��

He smacks a fist against his open palm for emphasis.

FORBES: Mr. T's Softer Side