From The Jargon File (version 4.4.7, 29 Dec 2003) :
heavy wizardry
n.
Code or designs that trade on a particularly intimate knowledge or
experience of a particular operating system or language or complex
application interface. Distinguished from deep magic, which trades more
on arcane theoretical knowledge. Writing device drivers is heavy wizardry;
so is interfacing to X (sense 2) without a toolkit. Esp.: found in
source-code comments of the form ?Heavy wizardry begins here?. Compare
voodoo programming.