It's a self-updating, browsable treemap with deeper keyword breakdowns when you click. For example, as I write this, Apolo Anton Ohno just won silver after a crazy finish in the 1500. Click on the related rectangle, and see the related tags and recent tweets. Use the scroll up top to move through time. http://flowingdata.com/2010/02/15/watching-the-pulse-of-the-olympics-on-twitter/
It supports investigation into the relationship between how words are used within the messages of all the people using Twitter.
the data is retrieved a Venn Diagram is shown which illustrates the rate of tweets containing the search terms in the various combinations.