This is a demo. As you move the mouse, Scratch is not detecting every point along its path. When you move the mouse fast, notice that the space between points becomes bigger. Press space. Scratch isn't detecting every point along the path, but rather drawing a line from the previous mouse position to the current one. Usually this line is short enough that you can't tell it is a straight line. Instead it looks like a smooth curve. But these are in fact many small, straight lines.
