Hasta La Motion!

Don’t get me wrong, I’m trying to solve this issue for a week and still can’t find the answer. I understand how it adds in "over" operation (alpha of foreground object is inverted, multiplied with background and then foreground object is added).

For example the background color where the alpha equals 0 is blue, and the alpha there is 0. Why it’s not 0 in the end? Shouldn’t it be scaled down to 0? If we mult any color by 0, it’s 0. Why does the opposite happens?

Maybe the reason is fusion additive merge mode? It says it "simply adds pixels, assuming boths images are premultuiplied".

Please help!