Final answer:
In case of an ECU failure, turning the ENGINE SPEED switch off at high-power settings will likely cause a temporary power loss, which might then trigger the APR system to respond, depending on the aircraft's design and safety features.
Step-by-step explanation:
The subject of this question pertains to aviation engineering, specifically related to the behavior of an aircraft's engine when there is an Electronic Control Unit (ECU) failure and the ENGINE SPEED switch is turned off. In such a scenario, the most common response would be that the engine will experience a temporary power loss before possibly shutting down or being reset by other systems designed to handle such failures. The Automatic Performance Reserve (APR) system, if present, may activate to compensate for the power loss, but it is not guaranteed as the primary response to an ECU failure at high-power settings. It is imperative to consult the specific aircraft's operating manual for exact procedures and outcomes related to ECU failures.