Here is a good explanation of vsync by Sharky and AKWarp a while back. Refresh Rate = Hertz
"Vsync is a funtion of your video card. With vsync on the output of the video card to the monitor is "syncronized" to the refresh rate of the monitor. In other words the video card will not send a new "frame" to the monitor until the monitor has finished painting the last frame.
Normally your video card will generate frames faster than your monitor can refresh so with vsnyc off your monitor may get refreshed with a new frame part way through painting the previous one. Although this happens so fast as not to be decrenable with the naked eye it can show up as bluring of the image slightly. However vsync off usually increase frame rate some.
The thing about rubber bullets and vsync is, so people contend that with vsync off, some of your bullets may be "lost" by the FE if they are inflight (on the way to the target) when the video card sends a new frame to the monitor before the last was finished painting.
With vsync off you will see somewhat higher frame rates (can be significant depending on the monitor and video card) but a slightly less "smooth" display. With vsync on you display will be crisper but with a lower frame rate (again it may be a significant loss depending on the video card and monitor)
Having vsync on only limits the highest end frame rates your video can generate (by syncing it to the refresh rate <Hz> of your monitor). It does nothing to the low end when the graphics get heavy. In other words, vsync only limits the max fps, but does not alter the low end when things get hot and heavy.
If a given situation causes you to run at, say 15 fps...then that same situation will give you 15 fps whether you have vsync on or off."
