https://github.com/10gen/mongo/blob/20d7bc2e7fa226725978ce2be25aa24b64292d1a/src/mongo/db/repl/oplog.cpp#L1856-L1874

As per parent ticket, this code is hard to understand, and this comment is not accurate. The person who takes on this ticket should spend time reading over the code to understand it, and update the comment to reflect how the code actually works, so future readers can refer to it.