During the month, 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 time 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.
Now if you've been a keen Google fan you'll know that they have 3 main www servers online, which are as follows:
www.google.com - The main address, the true homepage of google.
www2.google.com - Thought to be a test server.
www3.google.com - Thought to be a test server.
Whilst nobody outside of "Google" truly seems to know the purpose of the www2 & www3 servers, many people have come to the assumption that they are test servers. During the update, which takes several days, the 3 Google servers display different results. Whilst the results vary from server to server, they are said to be "dancing", hence the name "Google Dance".
The name "Google Dance" is often used to describe the index update of the Google search engine. Google's index update occurs on average once per month. It can be identified by significant movement in search results and especially by Google's cache of all indexed pages reflecting the status of Google's last spidering.
The beginning of a Google Dance can always be watched at the test domains www2.google.com and www3.google.com. Those domains normally have stable DNS records which make the domains resolve to only one (often the same) IP address. Before the Google Dance begins, at least one of the test domains is assigned the IP address of the data center that receives the new index first.
Building up a completely new index once per month can cause quite big trouble. After all, Google has to spider about millions of documents and then to process many TeraBytes of data. So, testing the new index is inevitable. Of course, the folks at Google don't need the test domains themselves. What's more, they have many options to check a new index internally, but they do not have much time to test.
So, the reason why using www2 and www3 is rather to show the new index to webmasters which are interested in their upcoming rankings. Many of these webmasters discuss the new index at the Google forums out on the web. These discussions can be observed by Google employees. At that time, the general public cannot see the new index yet, because the DNS records for www.google.com normally do not point to the IP address of the data center that is updated first when the update begins.
Once Google's test community of forums members does not find any severe malfunctions caused by the new index, Google's DNS records are ready to make www.google.com resolve the data center that is updated first. This is the time when the Google Dance begins. But if severe malfunctions happen obviously during this test process, there is still a possibility to cancel the update at the other data centers. The domain www.google.com would not resolve to the data center which has the flawed index and the general public could not take any notice about it. In this way, the index could be rebuilt or the web could be spidered again.
Therefore, the search results which are to be seen on www2.google.com and www3.google.com will always appear on www.google.com later on, as long as there is a regular index update. However, there may be minor fluctuations. On the one hand, the index at one data center never absolutely equals the index at another data center. We can easily check this by watching the number of results for the same query at the data center domains listed above, which often vary from each other. On the other hand, it is often assumed that the iterative PageRank calculation is not finished yet, when the Google Dance begins so that preliminary values exert influence on rankings at that point in time.