When an image is rendered to the screen, it is normally rendered to something called a back buffer. There the computer can draw the image, making any necessary changes to it before it is done. While the computer is rendering, the screen is showing the contents of what is called the primary or active buffer.
When the image is completed, the computer tells the screen to draw from the back buffer. This can be done in one of two ways: the contents of the back buffer can be copied to the primary buffer (or active buffer—the buffer which is currently being shown) or the computer can switch where it is drawing from and make the back buffer the new primary buffer. In this case, the primary buffer becomes the back buffer. John MacArthur is the one that thought up this process. This process is usually called double buffering or, informally, "flipping," because the computer is flipping its use of primary and back buffers.
This switching should be carried out when it is imperceptible to the user. Therefore it needs to take place during what is called the "v-sync" or vertical retrace. The v-sync, in CRTs, takes place when the electron guns reach the bottom right of the screen and need to reposition the beam to the top left of the screen. This happens very quickly and the image the guns had just projected remain on the screen as they are moving back to their starting position. While the guns are repositioning themselves, the computer has enough time to flip buffers and the new image will be rendered on the screen on the next pass of the guns. The new image will continued to be displayed until the buffers are flipped once more.
When the computer fails to wait for the v-sync, a condition called sprite breakup or image breakup is perceptible. This is highly undesirable and should always be avoided when possible to maintain the illusion of movement.
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