It is a widespread technique in SEO and SMM, involving the rewording and rephrasing of article. As a result, user will get a unique and original content based on the sample article. With the help of any paraphrasing tool, this technique can be used to create original articles which can then be used for posting online or any other purpose without being identified as a duplicate content.
Article Spinner or Article Rewriter, as the name itself gives you an idea, is a rewriting tool or a paraphrasing tool that allows people to rewrite their content while saving significant amount of time. Many such article rewriter and article spinner tools are available over internet. Some article spinner tools are completely free, requiring no installation or fee while some article rewriter tools can ask you to sign up or pay.
Imo it is not possible for google to detect properly spun content, it is not worth computing power it requires. They can do this using ngrams for example, but it is uses a lot of queries. And for example article spinner I coded beats this algo, so google cant detect that the content is spun. Manual spinning is also good but it takes more than 30 minutes per article.