[xwiki-users] Questions about custom authentication

Hi, I'm in the process of evaluating XWiki and I'm trying to integrate it with our SSO. I've so far written a class that extends XWikiAuthServiceImpl and I'm seeing calls to checkAuth and showLogin, but I don't really understand the logic behind those different calls. checkAuth seems to be called very frequently with XWiki.XWikiGuest as the user even though I returned a valid XWikiUser after the call that followed the authentication. Is there a document that explains what the different methods in XWikiAuthService do and when they are called? Any help or pointer would be appreciated. Mathias.