back in

... the engine driver drew over the points and at once backed into the siding. He backed in steadily and stopped. The siding would not hold the train, and the guard hooked off five waggons, and the engine ...

Whatever you throw is your chance. I called five for the main, which is the out chance, and threw seven to it, which is the in chance. If I throw five first, I lose, and if seven I win. You can back me in by betting the odds, or you can back me out, by taking the odds, the bank answers either way.