I just drove past the back of a ATR 72 while pax boarding and noticed there was no APU exhaust outlet at the back, but there was power to the a/c with no external ground power. I tried looking for the source from the adjacent bay and saw exhaust plume coming from the No 2 engine.
I asked a colleague of mine( no one has ATR 72 rating here) who told me that this a/c uses the no 2 engine as an apu while "braking" its propellers. Is it really braking the propellers or does it use some form of clutching mechanism to let the turbine run free of the propellers ? And does the APU provide bleed air for the packs as well ?
A pretty ingenious idea I must say !
