Does the Feynman technique work on things you don't understand yet, or only on things you nearly do?

You're doing the first half and skipping the half that does the work. The technique is a detector, not a generator: writing the explanation is how you find the gaps, and then you go back to the source and mark every place where you built a bridge out of vocabulary instead of understanding. Those bridges are usually easy to spot in your own handwriting — they're the sentences with 'basically', 'due to' and 'somehow' in them. Circle every one, look each up specifically, and rewrite. Without that second pass you've just written a nice essay from your own confusion.

It works for concepts and it fails for procedures, and I suspect your six weeks contains both. If the exam asks you to explain why something happens, then write and check. If it asks you to actually do something — integrate by parts, balance an equation, work through a proof — no amount of explaining will get you there, because that's a motor skill dressed as knowledge. For those, closed-book problems under time pressure is the only thing that transfers, and the explanation technique will fool you badly because you can describe the method perfectly and still not execute it.

Explaining to an actual person beats explaining to a notebook or a rubber duck, because a person asks the question you skipped. I run this with students in pairs: eight minutes to explain one topic, and the listener's only job is to ask 'why' twice and 'what would happen if that weren't true' once. The three questions are always the ones the explainer had quietly avoided. It's also much harder to bluff out loud than on paper, which is precisely the value.

I spent an entire term producing gorgeous explanatory notes and got a mediocre grade, which stung because it was clearly a lot of work. Writing things out in your own words is pleasant and it feels exactly like learning, and the fluency of your own handwriting is the trap. What changed my results was closing the book and trying to produce the answer cold, being wrong, and being annoyed about it. Do the explaining, but make yourself do it without the source open.

Write it, then check it against the text and mark every hand-wave. The checking is the technique. The writing is just what makes the checking possible.