From the book 活着 (audio book, about 7 minutes 40 seconds into 第01集):
I translate this to:
Several [几个] women [女人] wearing [包着] headscarves [头巾] are [正将] pulling out [拔出来] cotton stalks [棉秆]; they [她们] frequently [不时] shake [抖动着] their bottoms [屁股] so the root's [根须上的] mud [泥巴] falls off [摔去].
I'm confused about 屁股 (pìgu) = "bottom". I've only ever seen it used to mean "bottom" (of an animal) in the sense of "arse" (and not "bottom" as in "lower part" as it also means in English). So I feel it's odd to think about the "bottoms" of the cotton stalks in this context.
However, in this sentence, if it means the women are shaking their respective bottoms, I don't understand how that causes the mud to fall off the cotton stalks.
Question: Does 屁股 refer to the womens' bottoms?