On my Santorin you have to put the key in and switch on (next to your ameter). The green light comes on. It sends a current to the alternator to excite it and start it charging. You should turn off when finished using it and put your gear lever in reverse to stop the alternator charging before switching on the engine.
I believe the diodes in each of the engine and alternator should prevent any ‘issues’, but that is the prescribed Amel method.
My knowledge of electrics is limited - someone with a better understanding may chip in here.
The Santorin prop shaft pulley is quite a bit bigger too, so will turn the alternator faster. There is a significant drop in prop shaft speed as soon as you turn the alternator on.