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bit.ly/NQpDhVI think that this explains why some blackhatters see some dance while carrying on some tactics like blasts, massive on page changes and similar optimization techniques. Google reacts by dropping the rank of those "documents" in order to check if the owner of the site changes something back, and if he/she changes to the previous state it could mark as "SPAM" that document. Of course, read the article and you will get a better aproach.
"For example, rather than just immediately raise the rank of a page when there have been some modifications to it, and/or to the links pointed to a page, Google might wait for a while and even cause the rankings of a page to decline initially before it rises. Or the page might increase in rankings initially, but to a much smaller scale than the person making the changes might have expected.
The search engine may monitor the changes to that page and to links pointing to the page to see what type of response there is to that unusual activity. For instance, if someone stuffs a page full of keywords, instead of the page improving in rankings for certain queries, it might instead drop in rankings. If the person responsible for the page then comes along and removes those extra keywords, it?s an indication that some kind of rank modifying spamming was going on."
