Never use that technique Black Hat. So no idea at all.

Black hat SEO refers to the use of aggressive SEO strategies, techniques and tactics that focus only on search engines and not a human audience, and usually does not obey search engines guidelines. Some examples of black hat SEO techniques include keyword stuffing, invisible text, doorway pages, adding unrelated keywords to the page content or page swapping (changing the webpage entirely after it has been ranked by search engines). Freelance SEO services