This pair is not a second set of vritti standing alongside the five vritti. In the oldest surviving commentary, Vyāsa describes klishta and aklishta as modes any vritti can take: the same memory can be detrimental or not, depending on what feeds it and what it leaves behind. This pair gives us a question to ask of whatever arises, and keeps practice from becoming a war on thinking. Some mental activity supports the path, and the practitioner's job is telling which is which rather than shutting everything down.