It discounts clicks that have very little chance of generating sales. According to Mercury news, it discounts up to 15% of the clicks.
http://www.mercurynews.com/search/ci_5502924 No wonder my cash has been dropping as my clicks increase
"In fact, Yahoo!’s click-through protection system proactively identifies and does not bill advertisers for between 12 and 15 percent of clicks on average. This includes not only click fraud, but also other types of clicks that Yahoo! believes should not be billed to advertisers." this is not the same thing as was stated in the linked article. in fact, these are 2 completely different statements.