I was watching an episode of ファニエスト外語学院 on judo.
At 6:26, the judoka remarked how his opponent ボビー could win him just by his horrid face, saying
I am interested in understanding why the 助数詞
本 was used in the expression, especially when it is used for small, long things. I looked up
一本取る in ディジタル大辞泉 , which gives the definition
柔道・剣道などで、完全に技が一つ決まること。 So I believe
一本 here houses the meaning 'in one go/shot/try', though I am not completely sure.