Turning off VSync can cause "tearing".
Normally, when Vsync is enabled, your video is prevented from updating the frame buffer while that buffer is active. Instead, the updates are done during the Video Blank or on the alternate frame buffer(s). If you disable VSync then your video card is free to potentially perform updates on the active frame buffer. If this happens then the frame buffer would contain part of one frame and part of the old frame. The RAMDAC just generates an image of contents of the active frame buffer so it displays both partial frames, thus the "tearing" effect.
I think this only becomes a problem when your framerates are much higher than the refresh rate, more than 2X higher. It depends on whether you are double buffering or triple buffering but when your application is attempting to generate frames faster than all the buffers can handle then you get tearing.
At least, that's what I believe is at issue.
I'm positive about partial frames in the active buffer causing the tearing. I'm just not so sure I have the details about double and triple buffers right.
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