I beleive that's the economy/standard switch. economy locks the transmission in to starting in second gear, while standard allows first gear starts if you floor it.
I beleive it's described in detail in the archives, try a search?
BenzWorld.org forum is one of the largest Mercedes-Benz owner websites offering the most comprehensive collection of Mercedes-Benz information anywhere in the world. The site includes MB Forums, News, Galleries, Publications, Classifieds, Events and much more!