[agumonkey]

Reminds me of graphics double buffering. Back in the days games would write directly in the video buffer, while the graphic chip would scan that same buffer and push the content on screen at the same time.

If your code is too slow (a complex effect, too many character at that point), you might not be done writing a full frame when the graphic chips starts to output the pixels.

This means your TV is now showing partly old and new state. Nothing important most of the time, it's only games, it's only a few ms of absurd information, people's brain can compensate. It is ugly to see though. You have that weird 'line' somewhere below.

Since people changed the structure a bit, with two (or more) buffers, the program computes the new image in one buffer B, while the chip shows another buffer A. When you are done with a picture, the chip will now scan B, while you can write in A. This means the output never shows partial frame anymore.