Hidden text basically works by setting body text the same colour as the background. This is typically like so, so, so, so, so. Search engines have been able to ad extra checks into their search engine spiders. So it is quite easy for search engine spiders to find the hidden text (look for a font that has the same colour tag as the background colour. Default background colour is white.).
Search engines do not like this sort of technique as in the past it has been used to spam the search engines. See the Hidden text page for an example of the extents that some webmasters have gone to. This is penalised or either banned by the search engines. This sort of thing is one of the many things that search engines will look for when indexing a site. So if your site isn't in the search engines, look for hidden text.