关于what引导的主语从句

关于what引导的主语从句
先看这样一个句子,In general,the tests work most effectively when the qualities to be measured can be most precisely defined and least effectively when what is to be measured or predicted cannot be well defined.
有些问题不确定需要得到证实吧.
(我自己分析的不知道是否正确),
the tsets work  most effectively .and least effectively.主句
 to be measured 做定语
what引导的主语从句,也就相当于the qualities that.
那为什么不能把what直接换成和第一个when后面的the qualities呢?如果能换的话第二个when引导的状语从句里面有2个动词,就矛盾了.
我的理解是否正确,这是考研试题的句子,有点小难.