Does 屁股 refer to the womens' bottoms in 几个包着头巾的女人正将棉秆拔出来，她们不时抖动着屁股摔去根须上的泥巴? From the book 活着 (audio book, about 7 minutes 40 seconds into 第01集):


  几个包着头巾的女人正将棉秆拔出来，她们不时抖动着屁股摔去根须上的泥巴。


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?
摔去根须上的泥巴 means to swing cotton stalks 棉秆 and hit ground heavily in order to remove the mud attached on the root. In order to make sure the root can hit the ground, they have to squat or bend their knees. By repeating the motion several times, the mud can be removed thoroughly. So, looking behind, their butts shake up and down.