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How much the surviving frame grows before the new one is drawn on top. 1 leaves it alone.

This is the difference between a smear and a trip. A frame that comes back slightly larger every time turns every mark into a tunnel rushing at the viewer, because the mark's own echo keeps expanding behind it. Below 1 the echoes fall inward instead, which reads as being pulled down a drain. Small numbers go a long way: 1.02 is already strong.