G-Sync and FreeSync introduce variable refresh rates. If your game is rendering at 40 frames per second, your display will update at 40 frames per second. If it starts rendering at 75 frames per second, your monitor will refresh at 75 frames per second. The monitor and graphics processor talk to each other, and the refresh rate constantly changes to be the ideal one to match the images being sent to the display. This eliminates stuttering, input lag, and screen tearing, resulting in a much more fluid image when playing PC games without the problems of V-Sync.