This is an AS3 mouse trailer example using the Blitting technique.
From Wiki on Blitting: “A classic use for blitting is to render transparent sprites onto a background.”
So instead of having 100′s of movieclips on the screen at the same time, you only have 1 bitmap object. This dramatically improves performance.
This also allows you apply filters to this bitmap image to achieve all kinds of fancy effects, such as this trailer using blur and alphaMultiplier (bitmap alpha control).