I have no experience with an auto prop. I don’t know if it folds up if you leave the throttle lever in neutral.
For the prop shaft alternator to work you must:
Switch off the engine and take out the ignition key
Put the gear lever in neutral
(You should hear the prop shaft turning quite fast)
Put the other key attached to the ignition key in the prop shaft lock (on the 12v panel above the galley) and turn it on
The prop shaft revolutions slow down and a green light comes on to show the alternator is charging
( both keys are on a welded key ring so you cannot have the engine running with the prop shaft alternator)
To turn off:
Put the gear lever in REVERSE, never in forward.
(This stops the prop alternator and the green light goes out)
Put gear lever in neutral
Start engine again.
As I say, I have no experience of the autoprop so you may need advice from SM owners.
I am guessing your boat originally had a fixed propellor and this was changed. The previous owner probably wanted to leave the original equipment on the boat.
SN 96 (Wintering in Caernarfon, North Wales)