This action was to clench either their right or their left hand into a fist.

这个活动是左手或右手握紧拳头。

youdao

Then, perhaps- for there was no foreseeing how it might affect her- Pearl would frown, and clench her little fist, and harden her small features into a stern, unsympathising look of discontent.

此时，珠儿或许就会——因为无法预见那魔障可能对她有何影响——攥起小手，紧皱眉头，板起面孔，在小肠上露出不满的冷冷表情。

youdao

By contrast, people who are defensive or angry may protectively fold their arms across their chest, clench their hands into a fist or tightly grip their arm or wrist.

FORBES: Body Language Savvy for Sales

This action was to clench either their right or their left hand into a fist.

ECONOMIST: Reading the brain