With an interlaced picture (which is what we're all seeing when we watch a crt t.v., video or a standard dvd player) each frame is made up of 2 fields. When field 1 is created odd (1,3,5,7,9 etc.) scan lines are drawn. When field 1 is complete field 2 is created (scan lines 2,4,6,8 etc.) hence we have an interlaced picture. Imagine a horizontal line moving from the top of the screen to the bottom. As it moves it exists at a given position when field 1 is drawn but has moved downwards by the time field 2 is drawns. The resulting filcker, or shimmering effect which can be seem at the edges of the line is twitter.