Obviously the modern turboprops have small APUs, but there really were no such beasts in the early days of the Friendship, Viscount and the HS748 40 years ago. The HS748 had 4 meaty batteries for DC starter motors on the RR Dart turboprops. Although having a 12 foot, 4 bladed prop, it was startable as long as you didn't waste battery power on the ground. So the technique was to configure the aeroplane for departure on the taxi in, and as you shut down to kill the battery switch and be electrically dead until start up time. The prop brake was purely used to avoid damaging the propellor leading edges with passenger heads- the Dart could not run with the prop braked.