If you're playing a game that your system can really only reliably render at around 40 FPS, with VSync enabled on a 60Hz display, you would be capped to 30FPS. Lets say periodically, in small rooms within the game, your rig can actually muster 62FPS. Traditional VSync would jump you up to a 60FPS lock, only then to coldly throw you back down to thirty as soon as rendering is taxed again to anything 59 or lower. To mollify these brief increases in framerates, Smooth VSync will only sync to 60 if the system has been able to render at or above 60 for an extended and substantial period of time. The supposition is that a player would prefer a sustained 30 FPS experience, rather than one sprinkled with ephemeral glimpses into the greatness that is 60FPS, only to have it taken away, in a fashion that could lead to perceived stutter.

There is no difference between this syncing protocol, and standard VSync IF the system is attaining 60FPS or higher constantly. It is more akin to an Adaptive VSync, except one that does not allow tearing at all, and instead monitores whether VSync or 1/2 Refresh Rate VSync should be employed.