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。

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

youdao

玩家们把拳头握在手掌中，倒数3、2、1后伸手做出三种手势中的一种。 三种手势分别为：一只拳头（代表石头），伸开的手掌（代表布），以及两根手指（代表剪子）。

Players smack their fists into their palms and count to three before making one of three hand signals: a fist (rock), flat hand (paper) or two fingers (scissors).

youdao

杏树之上耸立着三十株庄严的棕榈树，它们蕨状的叶子如伸开的手指般在星空下散开。

Above the apricot trees towered thirty majestic palms, their fernlike fronds splayed like open fingers against the starry sky.

youdao

伸开你的手，确保你的手掌和我的手掌贴在一起，然后把你的大拇指扣在我的手背上，同时用其他手指轻轻地握紧我的手，很快地上下摇动两下后就松手。

Extend your hand, make sure the web of your hand makes contact with the web of my hand, close your thumb over the back of his hand; give a slight squeeze with your fingers; two quick pumps and let go.

youdao

伸开你的手，确保你的手掌和我的手掌贴在一起，然后把你的大拇指扣在我的手背上，同时用其他手指轻轻地握紧我的手，很快地上下摇动两下后就松手。

Extend your hand, make sure the web of your hand makes contact with the web of my hand, close your thumb over the back of his hand; give a slight squeeze with your fingers; two quick pumps and let go.

youdao