There does not appear to be any documentation on how to use Method in Eigensystem, or at least not in Version 9.0.1 of Mathematica (unless I'm looking in the wrong place). The documentation page for Eigensystem just gives a single example of using an Arnoldi shift as
Method -> {"Arnoldi", Shift -> -4}
and the internal implementation page at tutorial/SomeNotesOnInternalImplementation#5107 just says that Eigensystem uses an ARPACK Arnoldi solver for iterative computing of the first $k$ eigenvectors.
The documentation page on Method likewise makes no mention of what options are available for use in eigendecomposition problems. Is there any documentation on what possible syntax options are available for Method in the context of Eigensystem?