EMZETT.Sync (Display Sync)
In short: The timing coordination between the graphics card’s frame output rate and the monitor’s refresh rate, so images are displayed smoothly and without visible tearing.
In more detail: Without synchronisation, “screen tearing” can occur, when the monitor is already showing the next frame in the middle of building up a new one. Technologies like V-Sync, G-Sync or FreeSync align graphics-card and monitor timing in different ways.
In Depth
Without synchronisation, the graphics card and monitor work completely independently of each other: the graphics card renders frames as fast as it can, while the monitor updates on a fixed time cycle (its refresh rate). If a new, fully rendered frame arrives in the middle of the monitor’s update process, the top part of the screen still shows the old frame while the bottom part already shows the new one — visible as a horizontal “break” in the image (screen tearing), particularly disruptive during fast camera movements in games.
Older solutions like V-Sync prevent tearing by forcing the graphics card to output new frames only exactly in time with the monitor — this reliably eliminates tearing, but can cause noticeable input lag if the graphics card could render faster than the monitor allows. More modern, adaptive approaches like G-Sync (proprietary) or FreeSync/Adaptive Sync (cross-vendor standard) reverse the principle: instead of tying the graphics card to a fixed monitor cycle, the monitor dynamically adjusts its refresh rate to the frame rate actually delivered by the graphics card — tearing is avoided without the added-lag downside of classic V-Sync.
Variable refresh rate range
Adaptive-sync technologies only work within a certain frequency range (e.g. 48-144 Hz) — if the graphics card’s actual frame rate drops below this range (say, during a particularly demanding section of a game), synchronisation no longer works reliably, and image errors can reappear. Some monitors solve this via “low framerate compensation” (LFC), which duplicates individual frames as needed, to artificially stay within the supported range.
G-Sync Compatible as a bridge
Originally, real G-Sync required an additional, proprietary hardware chip in the monitor itself, which made these monitors noticeably more expensive than FreeSync models. With “G-Sync Compatible”, the graphics card manufacturer later introduced a certification allowing many already-existing FreeSync monitors (without the proprietary extra chip) to still be used with its own graphics cards — an example of how an initially closed standard can gradually converge with the more open competing standard.
See also: V-Sync, Monitor, Graphics Cards