Google updates its data time to time to fix websites position in search engine on a particular keyword. And thus some sites position go up and down. This process is called Google dance.

Thanks
Franky

I have seen this dance for my websites, boy seems like freaky stuff one minute you are on the first page second minute take you off to second or even fifth some times.

Google sends out robots (spiders) to crawl the web and archive every website it finds. These archived websites will be presented in the next update of the Google index (database). The update process is initiated every 30 days or so, although on some occasions there have been more than 30 days between updates.
Skill games
paddy power games
bluesquare games

Google Dance has often used to explain the index update of the Google search engine. Google's index update occurred on standard once per month. During an index update there was significant movement in search results and Google showed new backward links for pages.