How Walcha learned to memorize organ music?

My organ professor, George Ritchie at UNL told me a story of how his professor, the great blind German organist and composer Helmut Walcha memorized organ pieces (especially polyphonic compositions). I made a video recounting this story. Feel free to make an experiment of Walcha's method with one of your favorite organ compositions. Dr. Ritchie also talks about Walcha's memorization method in the section of the documentary "Desert Fugue".