Google’s backwards compatible is being broken, with following two effects: First is, called PageRank sculpting, a “power SEO” technique, and the second with bigger impact may get thousands of sites, into violating of Google’s rules on paid link without knowing it. In both cases, the changes may require site owners to alter their web sites not because they were “chasing the algorithm” but because they were following Google’s own rules and instructions.
In general, each web page that Google finds has some degree of importance called “PageRank” that the page can pass on to other pages. Links from that page to other pages are how it passes that importance along. With PageRank sculpting, you can block some of the links on your web page (using nofollow attribute or some other means) from getting any authority. Perhaps you have a lot of navigational links to other pages inside your web site. Rather than spend authority money on these pages, you might prefer to spend it on a smaller number of important pages that could use a boost.
Full Article: SEL blog