I'm having a little problem understanding what you wrote, but I'll take a stab at an explanation that might address your question.
APU air does not get inside the engine you are starting, it only drives are tiny turbine in a starter connected to the engine gearbox.
This external starter motor is connected to the compressors and turbine through the gear box and spins the engine compressors just fast enough to allow fuel to be burned in the combustor