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.


The Free On-line Dictionary of Computing (30 December 2018):

heavy wizardry

   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.  Especially
   found in source-code comments of the form "Heavy wizardry
   begins here".

   Compare voodoo programming.

   [Jargon File]