I do know it as a criterion for journalism, although why it should be so I've no idea. Maybe they want the top half for scribbling instructions for the typesetter, in the old pre-electronic days?
But I don't think it matters for submitting MS to publishers and agents - just seems like a waste of paper to me.
Gawd, I have never heard this before...sorry to be such an ignoramous, but what do people mean exactly about chapters starting halfway down a page? I've never written a piece of journalism that way either..