This is where a gentleman is having relations with a lady who is so large that he can't see what he's doing.
Julie was so fat that I was just ballast shovelling!
A phrase to describe a wife or girlfriend who doesn't like being driven around at high speed, berates your driving or generally gets scared in the passenger seat.
"Went for a blast around the country lanes, would have been more fun without the Screaming Ballast nagging me too slow down...."