I haven't experimented yet, but I think it is as follows.
When in 'S' mode, the transmission will hold gears longer when accelerating, and downshift quicker when braking. It will only redline a gear if the pedal is floored or if you initiated manual shifting with the rocker switch and haven't upshift. It will not let you stall out. Once you initiate manual shifting with the rocker switch (and transmission in 'S'), you have to shift back to 'D' to stop the manual override.
If I'm wrong, please correct me!