Meaning of 一挺 in 雪人大肚子一挺，他顽皮地说：“我就是冬天。”
This dictionary definition explains the use of 一:

表示猛然发出某种动作或突然出现某种情况
E.g. 往起一站 / 右手一挥 / 眼前一黑。

Basically, this use of 一 expresses the suddenness of an action either intentionally or unintentionally.