Barbara Tuchman recommended using the smallest sized index cards possible to force one only to "extract the strictly relevant" because copying by hand can be both "a chore and a bore".
In the same address in 1963, she encourages "distill[ing] from the very beginning, to pass the material through the grinder of one's own mind, so to speak." This practice is similar to modern day pedagogues who encourage this practice, but with the benefit of psychology research to back up the practice.
This advice is two-fold in terms of filtering out the useless material for an author, but the grinder metaphor indicates placing multiple types of material in to to a processor to see what new combinations of products come out the other end. This touches more subtly on the idea of combinatorial creativity encouraged by Raymond Llull, Matt Ridley, et al. or the serendipity described by Niklas Luhmann and others.
When did the writing for understanding idea begin within the tradition? Was it through experience in part and then underlined with psychology research? Visit Ahrens' references on this for particular papers to read.
Link to modality shift research.