You clenched your fist and then you unclenched.

你捏紧了拳头又松开了。

I remember how Baba's hands clenched around the steering wheel. Clenched and unclenched.

我记得爸爸的手死死抓住方向盘，一会儿抓紧，一会儿放松。

He leaned forward staring at me, slowly unclenched his hand and drew it across his mouth as if his mouth watered for me, and sat down again.

他身体前倾，盯着我看，慢慢松开手，用手捂住嘴巴，好像为我流口水似的，然后又坐了下来。