>Hmmm...how does that work? Does it push the lines above it down?

Yes.

>Or does it pull the lines below that point up?

>IOW, does it change the bottom margin if lines are removed?

Yes and no. The height of the Page Footer decreases but detail band doesn't fill it. It just creates empty space. Seems like a bug to me.

Keep in mind, that I didn't test it thoroughly, so maybe I'm missing something and there's some way to get it work the way you would expect it to.