VSync
From Xtreme3D
VSync
Vertical Synchronization | Vertical syncOptional parameter behavior of the driver for the video card. Complimentary vertical synchronization means that after drawing the next frame, while shifting the buffers, the driver will be waiting for the next retrace the monitor, and only then will shift the screen buffers.
Image on monitors with cathode ray tube drawing a beam of electrons, which consistently отрисовывает row from left to right, then returns to the beginning of the next line (the delay of the horizontal sync), then отрисовывает the following line, etc. After the beam has got in the bottom right corner of the screen, he returns to the upper left corner (the time for which he returns, is called the latency of the vertical sync).
Why Do I need a vertical synchronization? The fact is that the delay time synchronization of the vertical retrace is ideal for switching screen buffers. If the switch buffers at any other time, the part of the image on the screen will belong to the old frame, and part is new. Because of this, the artifacts appear between frames - may become noticeable unpleasant jitter, and even at high FPS animation visually will not look smooth.
However, as in the vertical synchronization delay is done, the FPS will inevitably be less than a similar scene, but with the VSync off. This is sometimes unacceptable, for example, in a variety of graphic tests.