Smooth Vsync is a new technology that can reduce stutter when Vsync is enabled and SLI is active.
When SLI is active and natural frame rates of games are below the refresh rate of your monitor, traditional vsync forces frame rates to quickly oscillate between the refresh rate and half the refresh rate (for example, between 60Hz and 30Hz). This variation is often perceived as stutter. Smooth Vsync improves this by locking into the sustainable frame rate of your game and only increasing the frame rate if the game performance moves sustainably above the refresh rate of your monitor. This does lower the average framerate of your game, but the experience in many cases is far better.