Picture a pocket knife closing: the blade swings on its hinge until the two halves lie flat against each other, with a click. It is a fast, sharp fold at one single point. One way to hold jack-knife is to keep that closing knife in view — first the object itself, then anything long that folds the same way, whether it is a in the air, a body on a mat, or a truck on ice.
可以想成一把折疊刀正在合起來：刀身沿著轉軸轉，直到兩半貼在一起，喀的一聲。那是一個又快又利的折，而且只折在一個點上。jack-knife 的各種意思，一種想法是都站在那把刀旁邊——先是那把刀本身，再來是所有會用同樣方式折起來的長東西：空中的跳水選手、墊子上的身體、結冰路面上的連結車。
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