Glad I've got this sorted out because I intend to use S from time to time.
From 'D' to 'S' you need to use the button but not from 'S' to 'D', just push the lever forward.You do not need to close the throttle while going in either direction.
'S' is very useful, obviously for overtaking, but also for joining a road from a side road and navigating roundabouts for instance.
When you select 'S' the revs will rise by about 1000 and the car will become more responsive. When you open the throttle the revs will rise more readily with a corresponding increase in engine noise. Don't worry the engine will not blow up, the red line on the rev counter doesn't start until 6500.
I use 'S' regularly and still return 48mpg.
Vic.