Autos - stick in D, ignore til you stop, then back in P. S is not much use imo when you have manual override (i.e. the paddles). For me, in S, it holds a nice high engine speed to keep the turbo spinning say 2,300rpm, but then when you floor it it pauses to slam it down two gears to get you at 4,000rpm. Well changing down two gears puts you on the same shaft so you get a second's pause, which defeats the object of sport mode.
Paddles useful when trying to drive fast on twisty roads; when preparing to overtake; or sometimes when towing.
I think in D, you can tap down a few gears with the paddles ready for an overtake then you are in manual mode for something like 20 seconds then it reverts to D. If you want manual control indefinitely it's something like press them both.
I don't have paddles, I have to move the stick to get manual mode, and this doesn't do the manual for 20 seconds thing, which would be quite useful when preparing to overtake.