The name " Google Dance " has often been used to describe the index update of Google search engine. During this period results can differ wildly.
The progression of a Google Dance could basically be watched by querying Google's data centers.

During the Google dance servers are updated individually from one to another. The data centers do not receive the new index at the same time. The new index is transferred to one data center after the other. If you check the results from one server to another during the update they will differ, and it changes as each server is updated, hence the term, Google Dance.

The reason people are interested in the dance is you can get an idea of your future google keyword ranking by checking the other servers as they are updated before the main server google.com. If you have added new pages and want to know if they have been indexed, checking google data centers during the dance will tell you.

AgentWebRanking let you check out what is happening with your site's position in Google data centers