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I think the theory is that (this close to an election) Jeremy would resign and (with no time to consult the membership) a 'more electable' leader would be parachuted in from the smoke-filled back room.
There would obviously be a promise to the membership that it would be properly ratified after the election.
It's a 'no downside scenario' - if they win, then the members would be happy to endorse the new leader, if they lose, then there was going to be a leadership contest anyway.