jdg78 I got it from this book:
Piano Technique (Dover Books On Music: Piano) by Gieseking and Leimer.
(Apparently, the Amazon link does not show. Sorry about that). EDIT: sometimes it does show. 🤷🏻♂️
But just a small part of it is devoted to the memorization approach (and it's a pretty difficult read, I think). It was translated from the original German version.
But, to cut a long story short: the Leimer/Gieseking approach to memorizing is essentially to memorize away from the piano, and not even begin to play the material until it has been memorized. The memorization itself takes place via reading the music while at the same time imagining the playing process; i.e. visualizing (and imagining the feeling of) all bodily movements (particularly fingers and hands, of course), and "hearing" the music in the mind. Having a good solid understanding of the music is definitely beneficial during the "imaginary playing".
Personally, I use a down-scaled version where I only memorize a couple of measures at a time, before going to the piano to test my memory of those measures. I also tend to memorize the hands separately. Allegedly, Gieseking himself was capable of learning whole works using Leimer's and his approach, without being physically at a piano. I dream of getting there, but ... baby steps. 😀
I read the book years ago, thinking at the time it would be way to hard to actually follow the memorization strategy. But then last year I watched a Youtube video by a Polish pianist who demonstrated his approach to memorization, and that re-awakened my interest in improving in that area, and then I remembered Leimer/Gieseking, and for some reason just thought it would be possible to follow this, so long as I stuck with smaller parts of the score, rather than the whole thing: