Secrecy & patented?
As I understand it: the technique is patented; the precise parameters (known as a "reset vector") which Google uses to generate its particular rankings are secret. If these parameters were to become known, they could be used to manipulate the rankings.
Garrett A. Wollman | As the Constitution endures, persons in every snipped-for-privacy@csail.mit.edu | generation can invoke its principles in their own Opinions not those | search for greater freedom. of MIT or CSAIL. | - A. Kennedy, Lawrence v. Texas, 539 U.S. 558 (2003)