Well, assume you have a webpage that talks about cars. And you want to make sure that the ads showing will be as revalent as possible. Your page has links to medicine sites and forum sites and you do not want google to 'think' you have anything to do with medicine.. here's how you can do this..
Code:
<head>
whatever ..
</head>
<body>
<google_ad_section_start>
your texts about cars ...
<google_ad_section_end>
<google_ad_section_start>
text that google shall ignore, medicine for example ..
<google_ad_section_end>
</body> Good luck cheating.
Google legal cheats 
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