When travelling on a vessel with an enclosed car deck, when should commercial vehicles start engines prior to disembarking?

Please start engines when the stern ramp/bow visor has started to open. 

This is an important action in helping to reduce the levels of elemental carbon (soot) within vehicle decks of those vessels with enclosed card decks - namely MV Isle of Mull, MV Caledonian Isles and MV Isle of Lewis.