It is done by turning on the cloacking device Well, cloaking is when your text is invisible, you write your text in the same color as background. That way you could stuff more keywords but I think that is being penalized by G. p.l.u.r.
That's different from what I think cloaking. quote from wiki Cloaking is a search engine optimization technique in which the content presented to the search engine spider is different from that presented to the users' browser; this is done by delivering content based on the IP addresses or the User-Agent HTTP header of whatever is requesting the page. The only legitimate uses for cloaking used to be for delivering content to users that search engines couldn't parse, like Macromedia Flash. However, cloaking is often used as a spamdexing technique, to try to trick search engines into giving the relevant site a higher ranking; it can also be used to trick search engine users into visiting a site based on the search engine description which site turns out to have substantially different - or even pornographic - content. For this reason some search engines threaten to ban sites using cloaking.
yes this days some of them work, there main aim is they create doorway pages with huge number of keywords .some of this tools are not dected by google.and i found many sites this days which r in top 10 using some or other black hat traicks which is very bad
Identification whether the arriving visitor is a search engine spider or a human is usually done either by checking the visitors' IP address, or his User-agent string.