Apparently, most NVidia drivers will let you merrily scribble into the frame buffer when it is still posting the frame to the video display. The scriblling itself is not the problem, but the restart of the frame draw is, which occurs when the sofware has finished scribbling to the frame buffer.
This is what causes most graphic glitches, especially the flashing at the bottom of the screen.
By capping the frame rate, it slows down AH enough to keep this from happening, especially with fast CPU systems.