If they're moving the selector to ON on the ground then it expedites the APU start when START is selected. Therefore, the APU can be started later hence it saves a few kilos of fuel.
Seems illogical to me.
Fuel is not going to be used until the starter motor has cranked the APU to a certain rpm. Cranking won't begin until the door is open. If you went from ON to START or OFF to START at the same time, you would save more fuel with the OFF to START because the APU would begin spooling up 5 or so seconds later.
Switch set to ON (but not start)... distractions occur... Now, did I remember to turn the APU selector to START? (Does the EICAS Memo message APU RUNNING appear prior to 95%rpm?).
Experts: What exactly is cold soak? What parts of the APU are affected by cold soak? (and what parts of the APU are you actually thawing out by leaving the inlet door open during a normal taxi to the gate?).