You want it on.
Your computer monitor refreshes in super fast moving lines . Normally your screen refreshes in vertical lines going up and down super fast. When you tun off the Vsync instead of your screen filling in every line in one pass, it will fill in every other line and then com back through a second time and fill in the lines it missed the first time. It may give you better FR but it will give you rubber bullets.
Edit: I think