If you see the immobilizer light on the dashboard when the engine shuts off you're tripping the immobilizer. You short the cables together, not the terminals. You don't want to short out the battery. Both cables should be disconnected from the battery.
IMHO, I would just disconnect the negative side and step on the brakes to drain any charge.
IMHO, I would just disconnect the negative side and step on the brakes to drain any charge.