I also always thought it was for "Sport Mode" for the reasons Ashen indicated.

EDIT:

Whoops!!

I stand corrected. I looked it up in the owners manual and it says:

Second (S)-Selecting Second shifts the transmission into a lower range of ratios for better acceleration and increased engine braking. Use Second when you are going down a steep hill or in stop and go driving.

Low (L)-To shift to Low press the release button on the front of the shift lever. Use Low to get more power when climbing and for maximum engine braking...