G-Sync. How does it work? Need help with a monitor.

[CoolUsername]

Hello everybody.
I am building a monstrous rig ( i7 5930K + Titan X SLI). Now how does G-Sync work? If the frame rate is higher than the refresh rate will it prevent tearing or if the framerate is lower than 60 will it make the experience smoother? Or both?

Let me get more specific:-
I can't decide what monitor to get the ACER Predator XB28HK ( 4k 60hz) or the Asus ROG Swift
(1440p 144hz) both are G-Sync. Please help. Also i am upgrading from a E7200 and int graphics. I am expecting a bit of a jump atleast.

 

[NEmpire95]

G-sync synchronizes the displays refresh rate with the GPU's render rate. It prevents tearing and stuttering and makes things look very smooth. Basically the second your GPU renders a frame it is shown on the monitor.

Also I would get this:

http://www.newegg.com/Product/Product.aspx?Item=N82E16824106004&cm_re=Acer_Predator_XB271HU-_-24-106-004-_-Product

instead of the ROG Swift.
It's 1440p, 144hz (OCable to 165hz) , ips, 27inch, g-sync, and it looks badass