You rotate the letters by a deterministic amount, by a fixed number of steps so that the end result is actually something that can be reversed.

你用确定的数量旋转那些字母，通过一个固定数量的步骤,这样，最终结果是可以把它,反加密出来。

哈佛公开课 - 计算机科学课程节选

When we have just a single bond in them molecule, you have all the free rotation you want, you can just spin it around, there's nothing keeping it in place.

当我们在分子力只有一个单键时，你可以随意旋转，你可以让它转起来，没有什么东西能固定住它。

麻省理工公开课 - 化学原理课程节选