I think you're talking about "area ruling". However it is not really about rapid change; more like "even" change.
Area ruling works like this. The "ideal" aerodynamic shape is like a tapered cigar. Think the fuselage of a Lockheed Constellation. The central concept is that the cross sectional area must taper, even if it is not shaped like a cigar. So you will have little area at the nose, growing towards the middle, and tapering off towards the tail. If you look at a 380 from below, you will see that the wing/fuselage fairing is quite "fat" in the rear. This avoids a drastic decrease in cross sectional area and makes the cross sectional area tapering smooth. The engines and flap track fairings play important roles in area ruling as well.
"There are no stupid questions, but there are a lot of inquisitive idiots." - John Ringo