This is an unfinished draft of one of the appendices to the unfinished draft of the book. I don't remember how far I got, but I believe everything currently stated in here is at least true, if perhaps not complete.
Now children, when a Foreground and a Background love each other very much, the Foreground inverts its Alpha channel and multiplies it by the Background. Then they add their color channels together and a new composite image is born.
You can take your compositing skills up a notch with a thorough understanding of the math behind Fusion's Merge and Channel Booleans tools. This article explains the various Operations available in the Channel Booleans node and the Apply Modes available in the Merge.