Good question, not sure I've got an answer. Just a term I've heard a lot, always sounded like it was doing something on the molecular level. Which I guess everything is. I think it's this. When you stretch the milk, you're trying to (about) double it in volume with tiny bubbles (microfoam). If the bubbles are too big, that's foaming the milk. I think. Lance will better explain it. But foam like that is the way it's done in most of Italy (from my limited experience). It's just a different way of doing it.