At work, I've been writing a word document to explain every part of a program I've written and how each element interacts with the rest. It's been very helpful to find things that don't make as much sense as I thought they did when I wrote them, and revealing old code that is no longer used anywhere in the application. And I've caught a few places where outdated .txt / .md / inline-doxygen documentation lies, too.

Writing a summary document really forces you to get to the nitty gritty and catch things that a casual review would miss.