A visual example of "Kill immediate if Zero Alpha".
Movie 1: The above movie shows a keyed person. Areas of white have zero alpha.
Movie 2: In the above movie when brushes fall into an area of the source movie that is completely transparent, they take 5 frames to shrink into non-existence because the "Death die rate" is set to 5.
Movie 3: Instead of making the death rate for brushes be only 1 frame, which would cause any brushes on the performer herself to "pop off" when two brushes were determined to be too close together, we check the "Kill immediate if zero alpha" setting. In this way, brushes internal to the area being painted won't pop off, but brushes outside the area to be painted will disappear immediately.