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Yes, S is Sport Mode. In D-mode the gears shift automatically at lower revs to deliver more economy and slightly less power performance. In S-mode the gears don't automatically shift until the revs are approaching towards the red and so your power performance is optimised. Reducing speed in S-mode optimises the engine braking as you would expect.
Both D and S modes can be instantly overridden by using the paddle shifter (or nudging the floor stick). It will revert back to the mode you were in (D or S) if you don't action another shift within 30 seconds. If you want to return to D or S within the 30 seconds, simply hold a paddle shifter for a second or two.
S-mode can be a bit frantic but can be very useful in some circumstances such as a very busy multi-junction roundabout.
I use D only for urban 20/30/40 mph driving or for traffic queue crawls. When my shoulders are level with the GLF (National Speed Limit) sign I shift to M and Go Like Feck! Then I think and shift gears in M exactly as I would a conventional manual.
DSG has a learning curve before it becomes fluid but I simply wouldn't buy another car without DSG or its equivalent. It's the best of both worlds in my opinion.
Hope this helps