Hi
When there engine fire or damage fire push button of that engine is pushed. If you use cross bleed the smoke fumes from that side will enter in the aircraft. As the APU bleed is connected to the left of cross bleed, if no.2 fire push button is pushed you can open APU bleed with cross bleed closed to supply no.1 pack and not the other way around. If no1 button is pushed APU bleed cannot be used as the cross bleed should not be opened.