Mark’s right, there is no APU on the ATR. The prop brake is on the right hand engine and when it is operation it is referred to as Hotel Mode. The Saab 340 has the same arrangement however most operators remove it as the weight advantage is in the order of 2 adult passengers!

By the way, it’s not just the P&W engines that need to do this but any free turbine engine (a turboprop where there is no mechanical linkage between the engine and the propeller).