The FMC gets the signal from the pack flow valves, engine and APU bleed valves to calculate target N1 for the engines.
With engines shutdown, pack valves will be closed (electrically controlled, pneumatically operated), until some bleed air is introduced. Moving the APU bleed air switch to ON will provide air to both packs, hence target N1 value will decrease, to simulate the same condition when the engines are running with packs in auto.