Hi,
Yes, it IS the APU you are protecting.
The engines have a modulating/shut-off valve to protect it from reverse flow, but the APU bleed air valve is 'dumb', and will not be aware of any reversal of airflow.
All the APU knows is that it's RPM is above 95%, and the bleed switch is commanding 'open' - so it does!
Going much above idle with the main engines has the same effect as turning off both packs, from HIGH, at the same instant, whilst on APU bleed, it creates a large back pressure, which can stall the APU, with a resulting overtemp, and auto shutdown!
Cheers
