This page by Nicolas T. Courtois mentions Geometric Generalised T' Method. It is described as
an advanced geometric algorithm, never published, for finding extra linearly independent equations at a given degree, which would normally be found by Gröbner bases techniques, but at a higher degree(!) with much higher memory and with much higher complexity. So in practice, this algorithm has no competitors in producing such equations on current computers.
The other page of the same site mentions T' method (not geometric and not generalized) as one of the steps of the AES attack and it even says that it is described in several papers (but sadly without any link):
In general, it is likely that in the future, the best attack on AES will be of the following form:
1. Use the equations over GF(256).
2. Apply one of the versions of XSL.
3. Apply a final step. The so called T' method, described in several papers, may or may not be sufficient.
So, my questions are: What is the T' method? Where it is described? What could be its geometric generalization?
Will be grateful for any reference or clue.