Secondary functionality which AMD are keen to explore with FreeSync is the behavior whilst V-Sync is enabled. Nvidia G-Sync always limits the frame rate to the upper boundary of the monitor's frame-rate envolope, generally 144 FPS. FreeSync by contrast alters its behavior depending on whether V-Sync is enabled or disabled, limiting frame rate to the upper bounds of the envelope is the former or letting it exceed it if the latter.
That can be a little confusing, so we once again took to Tomb Raider to demonstrate this behaviour. Rather than once again rely on the canned benchmark we took to 60-second snippet of gameplay, cranking down the image quality settings until the frame rate naturally crossed the 144fps boundary that's in effect for the Acer XG270HU.
As you can see, when V-Sync is off FreeSync operation is like any other monitor - it will attempt to render the frames for the monitor to display as fast as it can, likely exhibiting some tearing. However when V-Sync is on the frame rate is limited to the upper frame rate boundary, in this case 144 fps.
When you limit the frame-rate in this fashion you still have one of the same downsides of V-Sync at 60Hz: increased gameplay latency. In effect your GPU can render the frame and be part way through the next one before the frame can be displayed on the monitor, and that time spent part-way rendering is the effective additional latency you introduce. At 144Hz and above it won't be be particularly significant - the time spent rendering a frame is less than 7ms, and additional latency would only be a fraction of that.
Where it does become interesting is with other FreeSync monitors currently in the pipeline that top out at 70Hz. At those frame rates render times are hitting double-digit milliseconds (~14ms @70Hz) and so additional latency would begin to encroach on times which give peripheral manufacturers cold sweats. Recall that gaming mice promote 1ms response times with 8ms being the norm (USB normally polls at 125Hz), so if you're approaching 4ms added latency due to V-Sync at the top end it will start to add up.
Even in this best case scenario disabling V-Sync at the top-end will be of most value to competitive gamers rather than us mere mortals. Nonetheless this flexibility is a definite plus for the technology.
