Nothing. Both charged batteries have the same potential, so no current will flow. When the engine is cranked, the jumper connection will provide a lot of resistance, so the built-in battery will do most of the work.
After the engine is started, the alternator will recharge both batteries. The built-in battery will receive most of the current because it did most of the work. Both batteries will equalize again and current will stop flowing again.
Contact Person: Ms. Alice