Here is an overview and brief explanation of the link building related patents that Google has filed and may use for ranking sites.
Rank-Modifying Spammers Patent
In a nutshell: Google may / may not delay or falsely change the ranking of a page that has changed its ranking signals (content, links) in the short term.
At the same time, it may / may not follow other signal changes after the first rating changes. Ie. that the actual re-ranking of the page may also be delayed. The goal is to confuse spammers of all kinds.
Source and credits: http://www.seobythesea.com/2012/08/google-rank-modifying-spammers-patent/
In a nutshell: The quality of a link on a page can be estimated by the likelihood of a user clicking it. Links in the main content of a page are more likely to click than a footer, for example.
Other factors that affect the probability are, for example, the font size of the link, the order in the list of links the link is in, the type of link (tex, image), the destination of the link, and more.