Notice that “which” is relative pronoun and hence can refer to a noun entity only. However, in general English, for example in newspaper articles, more often than not, “which” is used to modify the preceding clause, as in the example presented by you:
It is undetectable and hard to understand, which leaves people feeling powerless.
In this sentence, “which” is modifying the preceding cause. It is accepted in general English writing. However, GMAT does not accept this modification. In GMAT, “which” must always refer to a noun entity, in most cases, the preceding noun entity. In our SC course concept namely "Modifiers - Relative Pronouns", we discuss in details the correct usage of relative pronoun modifers including "which".
Hope this helps.