Starting a jet engine.
MD-100,
If you use the APU/Ground air to drive a small highly geared turbine connected to the main engine gearbox you get more revolutions for each pound of air.
The APU would not have enough air to turn the main compressor.
Think of how much air is going down the intake to turn the compressor when you are doing a windmill start after an inflight shut down!!
Speedbird 48.