Except a software/website is doing the spinning, I can't believe it's done manually.
If rewriting should be done en masse, there are paid services, which produce quite readable texts. But fortunately, there are the same services for detecting plagiarism, and on the detection of paraphrasing of texts. Free versions do not do deep searches, but again, if it is very important, then for such services you can pay. And plagiarism will still be discovered.