What is the Geometric Generalised T' Method?
T' method was introduced in the paper Cryptanalysis of Block Ciphers with Overdefined Systems of Equations by Nicolas Courtois and Josef Pieprzyk (see section 6.1 and Appendix E).

It is a part of XSL attack on block ciphers (such as Rijndael and Serpent).

During XSL attack cipher is represented as a system of multivariate quadratic polynomials and the goal of T' method is to derive more linearly independent equations form it.

Don Coppersmith criticized this attack but noted that


  The method has some merit, and is worth investigating, but it does not 
  break Rijndael as it stands.