When switching, do this only at the end of a chapter, not in media res (in the middle of action).

Also summarize the last thing that happened/got explained for an easy refresher the next time you get back.

Bib-Card? Potentially Marginaelia? Feeling more like a dedicated notebook for this. Need to work out.

Vashik does this summary of a chapter on index cards... Useful to do in a Zettelkasten, or too much effort?