A practical test of understanding
Feeling familiar with an explanation does not necessarily mean you understand it. A more reliable test is whether you can use a minimal example to explain the input, process, and output.
Take caching. You do not need to start with complex consistency protocols. Imagine a sheet of paper recording questions and answers. When a question repeats, look up the answer.
Look for the conditions under which it fails
This simple example immediately raises new questions. When does an answer expire? What if nothing is recorded? What happens when two people change the record at once?
These are not unrelated complications. They define the concept's boundaries.
Keep a record of changing understanding
A useful note can contain four things: the original question, a minimal example, the conditions where it works, and the questions still unresolved.
When reviewing, cover the explanation and reconstruct it yourself. The places where you get stuck say more about what to do next than the number of times you have reread the page.