Why is the engine frame-rate limited?

[Eigenmusic]

If it limts my frame-rate to 60FPS, and my refresh rate is 60, then it's synched.

Why don't you explain how it's not?

And I don't mean Google links, I want you to explain how it's different. Can't wait to hear it....

Ok, I'm not the person this was directed at, but there's a simple answer and it relies on an understanding of the term "sync"; in this context, this word is short for "synchronized". In engineering terms, this means both the frequency and PHASE are the same in two signals. Two signals can be the same frequency, but may not be the same PHASE; in such a situation, the signals are "out of phase". This is exactly what is happening when your frame rate is locked to 60fps and v-sync is disabled.

Now then, a standard LCD monitor will usually refresh its display at a rate of 60 Hz (user selectable, etc); that is to say, it redraws the picture 60 times per second with nearly perfect precision in its timing. It draws pixel-by-pixel across the screen in the same way that a page of English text is read (left to right horizontally, and then to the next line down vertically). Your graphics processor draws frames at a variable rate, and we all like it to produce frames at least 30 times per second (preferably at least 60, but I don't want anyone whining about that).

When you enable "vertical synchronization" (aka v-sync), you are telling the graphics card to only send frames to the screen at the exact moment that the monitor is planning to start drawing the next frame (when the vertical line being drawn switches from bottom to top). If that timing is off, then the monitor may start drawing the next frame while it is still trying to finish drawing the current frame (60+ fps) or may start drawing a slow frame after the monitor has drawn the same frame multiple times (59 or less fps)! That is what causes "tearing"; half the image is one frame and the other half is another frame. With v-sync on, that won't happen! The graphics processor will only send a new frame to the monitor when it knows that the monitor is ready to draw a new frame, whether that means waiting for the last frame to be drawn multiple times or producing several frames and having to throw them out.

Now, dynamically speaking, there is a lot going on here, because the graphics processor may be able to produce 60+ fps at one moment, but slow to a crawl at the next moment. Also, obviously, there is a lot more to this on the technical side of things, but that's the basic idea of v-sync.