The dash top image includes transparency. To apply this, Stunts does what was fastest at the time: you first AND a mask against the background. The pixels that should remain from the background are coupled with bytes of value 255 and the pixels that should not show because the image will go on top are coupled with a 0. Then, you OR the actual image with the background. Because the background pixels were previously removed where the image is going to be, you get the combined image without superposition.
Now the thing is you can use a non-zero mask at places where you actually have a non-zero image pixel as well. This will cause the foreground image to combine with the background using an OR. Because OR is destructive (information is lost), it's very limited what you can get, but choosing the right colour, you may actually like how that destructive effect looks.
So if I understand correctly, Alan Rotoi is extending the dash top to reach the roof and is filling the parts that should be transparent with a non-zero colour. The areas where the light illuminates are filled with zero or with another colour that's less destructive (one that has fewer 1's and more 0's).
