The chain has to move up, before it can move off (to the side).
And it has to move sideways first to be lifted up by the chainring teeth
I was talking about "chainslap" though (on the subject of single-ringers losing their chain).
If it just moves up and down, it stays on the chainring. It only comes off when it moves sideways far enough for the top of the teeth to catch the side plates.
@Scilly.Suffolk started
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And it has to move sideways first to be lifted up by the chainring teeth