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Towards then end of the month google are rollingout their next index with all the new pages in it. They can be seen doing this on www2.goo. and www3.goo. So when the update is on (AKA "dance") then perforn a search on normal google then do the same with the 2 and 3 and you should see some differences.
If Google are happy with the new index they will move it over to the main URL.
To find out when the dance has begun = check WebmasterWorld :) or www2 and www3.
Cheers
How you know when it's THE update is when the backlinks change; that's why the big sites are watched on www and www2. Those are done once a month, and the PR is recalced monthly. The major regular crawl takes place at that time, which isn't the same as what happens all month.
Full explanation from Brett and GoogleGuy here:
[webmasterworld.com...]
The false alarms are a problem each month. The above is the only way to tell if an update has started (yahoo backlinks).
This months update probably won't happen for two more weeks.