Google's famous Panda punishes websites by ranking that hide their content to users. The rule is that the content must be open to their bots and the users who access it from the search engine, and it mustn't be hidden even partially. However, there is a way around that Google calls "First Click Free".
- Совместимость с XenForo:
Official Google Webmaster Central Blog: First Click Free for Web Search
Registration/subscription sites - News Help
So basically what the add-on does is, it allows the visitor be able to see the whole content if the request comes directly from the search engine(s) (also bots). However, users need to register and confirm their account if clicking any other link within the site. (See attachment)