Simple Directmedia Layer

I use C++ and SDL to make a 2D Sidescroller Game.
But I have problems with the "Tearing", it looks fucking ugly when move sprites.
I already activated "SDL_DOUBLEBUF" and "SDL_FULLSCREEN".
I also switched "Doublebuffering" on, at my graphics card configs.
I even tried "SDL_SWSURFACE"(software renderer) instead of "SDL_HWSURFACE"(hardware renderer).
And of cource I use SDL_Flip instead of SDL_Update.

What am I doing wrong?

YAY!! I finally found something that works. (After hours of google)

Before I SDL_Init() the video mode, I have to wright
SDL_putenv("SDL_VIDEODRIVER=directx");

Looks like it forces SDL to use DirectX, and so my Hardwaremode is finally enabled, and no shitty tearing anymore.

But still, would be good to know what that so is.
Does someone have an idea?