In an emergency operation, what must you do to shut down the torch while using the Portable Exothermic Cutting Unit?

To properly shut down the torch while using a Portable Exothermic Cutting Unit, releasing the torch oxygen lever is the appropriate action. Doing so interrupts the flow of oxygen, which is crucial for sustaining the combustion process. By stopping the oxygen supply, you effectively extinguish the flame, as oxygen is a critical component of the combustion reaction.

While other steps may be necessary as part of an overall safety protocol, the immediate action to cease cutting operations and safely shut down the torch involves controlling the oxygen supply. This ensures a quick termination of the flame, which is essential for preventing accidents and ensuring safety in emergency operations.