"of" is used to indicate something from which position or reckoning is defined.

这是一个完整的包含一个定语从句的复合句。

划线部分为定语从句。which 是关系代词（严格说是关系限定词），指代前文something。同时，作了后面名词的定语。

这是给介词 of 的一个用法下定义，所以加了引号，充当句子的主语。

关于 which 的这个用法，再举几例：

He lectured for an hour, during which time many sutdents slept.

他讲了一个小时的课，在这期间许多学生都睡着了。

It took two years for me to get better, during which time he spoon-fed me and did absolutely everything around the house. 

我过了两年时间才好起来，其间他喂我进食，并承担全部的家务。

《柯林斯高阶英汉双解学习词典》

They stayed with me for three weeks during which time they drunk all the wine I had. 

他们和我一起呆了三周，在这期间他们喝光了我所有的酒。