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

Pick the number of spiders.

确定蜘蛛的数量。

麻省理工公开课 - 计算机科学及编程导论课程节选