The 60+ year old marvel of technology that powers our flight is a nearly perfect match. Finicky, yes, but given the power, we get out of it’s few pounds, we can’t be terribly surprised.
When the piston moving upward, it’s expelling exhaust, but a plug of incoming fuel/air mixture also goes right out the exhaust. In an effort to recoup this, clever designers built the expansion chamber. It uses a reflective wave pulse to push the fuel part of that exhaust back in the chamber.
In the above GIF, notice a bit of green going out the exhaust then being pushed back in.
Image courtesy Wikipedia. It’s the most concise animation I’ve seen to date on understanding these things.