-- Adaptive VSYNC looks visually exactly like VSYNC ON whenever framerate matches refresh rate.
-- Adaptive VSYNC looks visually exactly like VSYNC OFF whenever framerate falls below refresh rate.
Adaptive VSYNC is simply a special mode of VSYNC OFF where the graphics drivers intelligently steers the tearing to be off the edge of the screen (via timing the frames), on a best-effort basis. So no extra frame buffers are used for Adaptive VSYNC than for VSYNC OFF. Adaptive VSYNC is kind of like an input-lag-reduced VSYNC ON, which is a great compromise for people who want the perfect motion fluidity of framerate=Hz of VSYNC ON but hate the input lag issues of VSYNC ON.
If you're always running framerates below refresh rates, Adaptive VSYNC will always look like VSYNC OFF, and there's really no difference in tearing. The pros/cons for your eyes basically occurs whenever you're sometimes at framerate=Hz and sometimes framerate lower than Hz.
With VSYNC ON:
...framerates that occasionally dip lower than Hz, will cause a sudden division of framerates (e.g. 120fps -> 60fps -> 40fps -> 30fps) accompanied by sudden annoying increases/fluctuations in input lag. Some people who really hate tearing, still prefer this setting.
With Adaptive VSYNC:
...framerates that occasionally dip lower than Hz, will cause tearing to suddenly appear, it just behaves exactly like VSYNC OFF. You won't get the sudden dramatic changes of input lag that occurs with framerate fluctuations with VSYNC ON.
That said, if your framerate is always below refresh rate, it doesn't matter if it's VSYNC OFF or Adaptive VSYNC since both behave exactly the same in that case (motion, tearing, lag). Adaptive is useful if you're often framerate=Hz (but not sustaining it at all times).
Edited by mdrejhon - 8/9/13 at 4:07pm